refactor(schema): make runs.task_id nullable, delete heartbeat workaround

The NOT NULL constraint forced heartbeat runs to synthesize a fake task
row (`[Heartbeat] X — Check for work`) so the FK could resolve. Three
layers of code — acquireHeartbeatTask, findReusableHeartbeatTask,
cleanupDuplicateHeartbeatTasks, plus the is_heartbeat flag column + UI
filter — existed to paper over one bad constraint. Remove the
constraint and the whole workaround vanishes.

SQLite table rebuild with data preservation. Live production DB, no
backup — migration is atomic, idempotent, and verified before DROP.

## Migration v35 (rebuild runs with nullable task_id)

Uses `fn:` form so it can:
- Snapshot `COUNT(*) FROM runs` before copy
- Walk `PRAGMA table_info(runs)` to detect drift columns (v34 declared
  a column set; any additive ALTER since v14 shows up here)
- Assert `COUNT(*) FROM runs_new == beforeCount` before `DROP TABLE runs`
- `PRAGMA defer_foreign_keys = ON` so run_events(run_id) FK defers to
  commit time, by which runs has the same id set under the new name
- Recreate all 6 indexes on the new table

If the assertion fails, the migration throws inside the existing
`transaction()` wrapper. SQLite rolls back the whole migration, old
`runs` table is untouched, `schema_migrations` never records v35. Next
app start retries cleanly.

## Migration v36 (drop tasks.is_heartbeat + synthetic rows)

Runs now carry `task_id = NULL` for heartbeat dispatches. The
`is_heartbeat` flag is dead data going forward.

1. `UPDATE runs SET task_id = NULL WHERE task_id IN (SELECT id FROM tasks WHERE is_heartbeat = 1)`
2. `DELETE FROM tasks WHERE is_heartbeat = 1` (synthetic rows, not real work)
3. Rebuild `tasks` without the `is_heartbeat` column (same defer-FK +
   row-count-assert pattern as v35)

## Code cleanup

- `RunRecord.taskId: string | null`; `createRun` / `cancelRunForDeletion`
  / `finishRun` guard every `UPDATE tasks` path with a null-check.
- `TaskRecord.isHeartbeat` removed; every SELECT / filter / UI reference
  dropped.
- Delete `acquireHeartbeatTask` / `findReusableHeartbeatTask` /
  `cleanupDuplicateHeartbeatTasks` — dead after the constraint lifts.
- Delete `src/main/orchestration/heartbeat-task.ts` (pure workaround
  constant file).
- `heartbeat.ts` creates runs with `taskId: null` when the agent has no
  assigned work. Turn context already supports nullable task
  (heartbeatFallbackSection); prompt surface unchanged.
- `validateTaskCreation` drops the `isHeartbeat` parameter — user-
  authored + automation-authored tasks are never heartbeat.
- Renderer: `run.taskId ? taskMap.get(...) : undefined` in Dashboard /
  RunCenter / RunDetail / RunQueue.
- Delete `tests/unit/heartbeat-coalescing.test.ts` (12 tests that
  exercised the deleted DB methods).
- Drop 1 isHeartbeat-bypass test from `task-quality-gates.test.ts`.

## Grep verification

- `isHeartbeatTaskTitle src/` → 0
- `acquireHeartbeatTask src/` → migration comment only
- `is_heartbeat src/` → schema.ts migration history only
- `isHeartbeat src/` → 0

Tests: 578/578 (was 591 — delta is 13 tests that tested deleted code).

src/main/database/tasks-crud.ts

@@ -33,7 +33,6 @@ export type TaskListByStatusesRow = {
   sessionDisplayId: string | null;
   costUsd: number | null;
   requiresUserReview: boolean;
-  isHeartbeat: boolean;
   startedAt: string | null;
   completedAt: string | null;
   cancelledAt: string | null;
@@ -170,23 +169,18 @@ export function buildTaskCrudMethods(
     },
 
     /**
-     * Returns actionable real work for an agent. Heartbeat tasks are
-     * excluded by default since they are synthesized wake-up placeholders;
-     * pass `{ includeHeartbeat: true }` to include them (heartbeat-loop
-     * callers that already filter by `is_heartbeat` do not need this).
+     * Returns actionable real work for an agent.
      */
     listAgentTasks(
       agentId: string,
       companyId: string,
       statuses: string[],
-      options: { includeHeartbeat?: boolean } = {},
     ): TaskRecord[] {
       const placeholders = statuses.map(() => "?").join(",");
-      const heartbeatClause = options.includeHeartbeat ? "" : " and is_heartbeat = 0";
       return readAll<TaskRecord>(
-        `select id, company_id as companyId, project_id as projectId, goal_id as goalId, parent_id as parentId, title, description, assignee_agent_id as assigneeAgentId, workspace_id as workspaceId, status, priority, active_run_id as activeRunId, session_json as sessionJson, session_display_id as sessionDisplayId, cost_usd as costUsd, task_type as taskType, is_heartbeat as isHeartbeat, created_at as createdAt, updated_at as updatedAt
+        `select id, company_id as companyId, project_id as projectId, goal_id as goalId, parent_id as parentId, title, description, assignee_agent_id as assigneeAgentId, workspace_id as workspaceId, status, priority, active_run_id as activeRunId, session_json as sessionJson, session_display_id as sessionDisplayId, cost_usd as costUsd, task_type as taskType, created_at as createdAt, updated_at as updatedAt
          from tasks
-        where company_id = ? and assignee_agent_id = ? and status in (${placeholders})${heartbeatClause}
+        where company_id = ? and assignee_agent_id = ? and status in (${placeholders})
         order by
           case status
             when 'in_progress' then 0
@@ -200,21 +194,19 @@ export function buildTaskCrudMethods(
         companyId,
         agentId,
         ...statuses,
-      ).map((t) => ({ ...t, isHeartbeat: Boolean(t.isHeartbeat) }));
+      );
     },
 
     /**
      * Returns the slim task projection (id, title, status, priority) for a
-     * single assignee, ordered by created_at desc. Used by the alternate
-     * me/standup builder in routes-agents-standup.ts. Heartbeat tasks are
-     * excluded so UI aggregations do not surface synthetic wake tasks.
+     * single assignee, ordered by created_at desc.
      */
     listAssigneeTasksForStandup(
       companyId: string,
       agentId: string,
     ): Array<{ id: string; title: string; status: string; priority: string }> {
       return readAll(
-        "select id, title, status, priority from tasks where company_id = ? and assignee_agent_id = ? and is_heartbeat = 0 order by created_at desc",
+        "select id, title, status, priority from tasks where company_id = ? and assignee_agent_id = ? order by created_at desc",
         companyId,
         agentId,
       );
@@ -222,9 +214,7 @@ export function buildTaskCrudMethods(
 
     /**
      * Returns the slim task projection for a set of assignees. Safe on
-     * an empty assignee list (returns []). Used by the alternate
-     * me/standup builder to fetch reports' tasks in one query. Heartbeat
-     * tasks are excluded.
+     * an empty assignee list (returns []).
      */
     listTasksForAssignees(
       companyId: string,
@@ -239,7 +229,7 @@ export function buildTaskCrudMethods(
       if (assigneeIds.length === 0) return [];
       const placeholders = assigneeIds.map(() => "?").join(",");
       return readAll(
-        `select id, title, status, priority, assignee_agent_id as assigneeAgentId from tasks where company_id = ? and assignee_agent_id in (${placeholders}) and is_heartbeat = 0 order by created_at desc`,
+        `select id, title, status, priority, assignee_agent_id as assigneeAgentId from tasks where company_id = ? and assignee_agent_id in (${placeholders}) order by created_at desc`,
         companyId,
         ...assigneeIds,
       );
@@ -247,7 +237,7 @@ export function buildTaskCrudMethods(
 
     /**
      * Returns the slim task projection used by the me/standup aggregation.
-     * Ordered by created_at desc. Heartbeat tasks are excluded.
+     * Ordered by created_at desc.
      */
     listTasksForStandup(companyId: string): Array<{
       id: string;
@@ -258,32 +248,26 @@ export function buildTaskCrudMethods(
       priority: string;
     }> {
       return readAll(
-        "select id, company_id as companyId, title, assignee_agent_id as assigneeAgentId, status, priority from tasks where company_id = ? and is_heartbeat = 0 order by created_at desc",
+        "select id, company_id as companyId, title, assignee_agent_id as assigneeAgentId, status, priority from tasks where company_id = ? order by created_at desc",
         companyId,
       );
     },
 
     /**
-     * Returns tasks filtered by status. Heartbeat tasks are excluded by
-     * default — they are synthesized wake placeholders and should not
-     * appear in the user-facing issue list. Pass `{ includeHeartbeat: true }`
-     * to include them.
+     * Returns tasks filtered by status.
      */
     listTasksByStatuses(
       companyId: string,
       statuses: string[],
-      options: { includeHeartbeat?: boolean } = {},
     ): TaskListByStatusesRow[] {
       const placeholders = statuses.map(() => "?").join(", ");
-      const heartbeatClause = options.includeHeartbeat ? "" : " and is_heartbeat = 0";
-      return readAll<TaskListByStatusesRow & { requiresUserReview: number | boolean; isHeartbeat: number | boolean }>(
-        `select id, company_id as companyId, project_id as projectId, goal_id as goalId, parent_id as parentId, title, description, assignee_agent_id as assigneeAgentId, workspace_id as workspaceId, status, priority, active_run_id as activeRunId, session_json as sessionJson, session_display_id as sessionDisplayId, cost_usd as costUsd, requires_user_review as requiresUserReview, is_heartbeat as isHeartbeat, started_at as startedAt, completed_at as completedAt, cancelled_at as cancelledAt, created_at as createdAt, updated_at as updatedAt from tasks where company_id = ? and status in (${placeholders})${heartbeatClause} order by created_at desc`,
+      return readAll<TaskListByStatusesRow & { requiresUserReview: number | boolean }>(
+        `select id, company_id as companyId, project_id as projectId, goal_id as goalId, parent_id as parentId, title, description, assignee_agent_id as assigneeAgentId, workspace_id as workspaceId, status, priority, active_run_id as activeRunId, session_json as sessionJson, session_display_id as sessionDisplayId, cost_usd as costUsd, requires_user_review as requiresUserReview, started_at as startedAt, completed_at as completedAt, cancelled_at as cancelledAt, created_at as createdAt, updated_at as updatedAt from tasks where company_id = ? and status in (${placeholders}) order by created_at desc`,
         companyId,
         ...statuses,
       ).map((t) => ({
         ...t,
         requiresUserReview: Boolean(t.requiresUserReview),
-        isHeartbeat: Boolean(t.isHeartbeat),
         startedAt: t.startedAt ?? null,
         completedAt: t.completedAt ?? null,
         cancelledAt: t.cancelledAt ?? null,
@@ -293,7 +277,7 @@ export function buildTaskCrudMethods(
     countActiveTasksForCompany(companyId: string): number {
       const row = getDb()
         .prepare(
-          "select count(*) as cnt from tasks where company_id = ? and status not in ('done', 'cancelled') and is_heartbeat = 0",
+          "select count(*) as cnt from tasks where company_id = ? and status not in ('done', 'cancelled')",
         )
         .get(companyId) as { cnt: number };
       return row.cnt;