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-status.ts

@@ -1,4 +1,3 @@
-import { randomUUID } from "node:crypto";
 import type { DatabaseSync } from "node:sqlite";
 import type { TaskRecord } from "@shared/types";
 import {
@@ -12,7 +11,7 @@ import { chooseBestTaskAssignee } from "./task-assignment";
 export function buildTaskStatusMethods(
   getDb: () => DatabaseSync,
   readAll: ReadAll,
-  transaction: Transaction,
+  _transaction: Transaction,
   deps: {
     addComment: (input: { companyId: string; taskId: string; authorAgentId: string | null; authorName: string; body: string }) => void;
     addActivity: (input: { companyId: string; actor: string; action: string; entityType: string; entityId: string; detail: string }) => void;
@@ -102,152 +101,6 @@ export function buildTaskStatusMethods(
       return { previousStatus };
     },
 
-    findReusableHeartbeatTask(agentId: string, companyId: string): TaskRecord | null {
-      const row = getDb()
-        .prepare(
-          `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,
-                created_at as createdAt, updated_at as updatedAt
-         FROM tasks
-         WHERE company_id = ? AND assignee_agent_id = ? AND is_heartbeat = 1
-           AND status IN ('done', 'in_review')
-         ORDER BY updated_at DESC, rowid DESC
-         LIMIT 1`,
-        )
-        .get(companyId, agentId) as TaskRecord | undefined;
-      if (row) {
-        row.requiresUserReview = Boolean(row.requiresUserReview);
-        row.isHeartbeat = Boolean(row.isHeartbeat);
-      }
-      return row ?? null;
-    },
-
-    acquireHeartbeatTask(input: {
-      agentId: string;
-      companyId: string;
-      runId: string;
-      workspaceId: string | null;
-      title: string;
-      description: string;
-      priority: string;
-    }): TaskRecord | null {
-      return transaction<TaskRecord | null>(() => {
-        const ts = nowIso();
-        const db = getDb();
-
-        const claimable = db
-          .prepare(
-            `SELECT id, status FROM tasks
-             WHERE company_id = ? AND assignee_agent_id = ? AND is_heartbeat = 1
-               AND status IN ('todo', 'backlog', 'done', 'in_review')
-               AND checkout_run_id IS NULL
-             ORDER BY
-               CASE status WHEN 'todo' THEN 0 WHEN 'backlog' THEN 1 WHEN 'in_review' THEN 2 WHEN 'done' THEN 3 ELSE 4 END ASC,
-               updated_at DESC, rowid DESC
-             LIMIT 1`,
-          )
-          .get(input.companyId, input.agentId) as { id: string; status: string } | undefined;
-
-        let taskId: string | null = null;
-        if (claimable) {
-          const claimResult = db
-            .prepare(
-              `UPDATE tasks
-                 SET checkout_run_id = ?, execution_locked_at = ?, status = 'in_progress',
-                     started_at = COALESCE(started_at, ?), completed_at = NULL, cancelled_at = NULL,
-                     updated_at = ?
-               WHERE id = ? AND checkout_run_id IS NULL
-                 AND status IN ('todo', 'backlog', 'done', 'in_review')`,
-            )
-            .run(input.runId, ts, ts, ts, claimable.id);
-          if (claimResult.changes > 0) {
-            taskId = claimable.id;
-          } else {
-            return null;
-          }
-        } else {
-          const inflight = db
-            .prepare(
-              `SELECT 1 FROM tasks
-               WHERE company_id = ? AND assignee_agent_id = ? AND is_heartbeat = 1
-                 AND status NOT IN ('done', 'cancelled')
-               LIMIT 1`,
-            )
-            .get(input.companyId, input.agentId);
-          if (inflight) return null;
-
-          taskId = randomUUID();
-          db.prepare(
-            `INSERT INTO tasks
-               (id, company_id, project_id, goal_id, parent_id, title, description,
-                assignee_agent_id, workspace_id, status, priority,
-                checkout_run_id, execution_locked_at, started_at, is_heartbeat,
-                created_at, updated_at)
-             VALUES (?, ?, NULL, NULL, NULL, ?, ?, ?, ?, 'in_progress', ?, ?, ?, ?, 1, ?, ?)`,
-          ).run(
-            taskId,
-            input.companyId,
-            input.title,
-            input.description,
-            input.agentId,
-            input.workspaceId,
-            input.priority,
-            input.runId,
-            ts,
-            ts,
-            ts,
-            ts,
-          );
-        }
-
-        const row = db
-          .prepare(
-            `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, task_type as taskType,
-                    started_at as startedAt, completed_at as completedAt, cancelled_at as cancelledAt,
-                    created_at as createdAt, updated_at as updatedAt
-             FROM tasks WHERE id = ?`,
-          )
-          .get(taskId) as TaskRecord | undefined;
-        if (!row) return null;
-        row.requiresUserReview = Boolean(row.requiresUserReview);
-        row.isHeartbeat = Boolean(row.isHeartbeat);
-        row.startedAt = row.startedAt ?? null;
-        row.completedAt = row.completedAt ?? null;
-        row.cancelledAt = row.cancelledAt ?? null;
-        return row;
-      });
-    },
-
-    cleanupDuplicateHeartbeatTasks(companyId: string): number {
-      const ts = nowIso();
-      const result = getDb()
-        .prepare(
-          `UPDATE tasks SET status = 'cancelled', updated_at = ?
-         WHERE company_id = ? AND is_heartbeat = 1
-           AND status IN ('done', 'in_review', 'todo', 'blocked', 'backlog')
-           AND id NOT IN (
-             SELECT id FROM (
-               SELECT id, ROW_NUMBER() OVER (PARTITION BY assignee_agent_id ORDER BY updated_at DESC, rowid DESC) as rn
-               FROM tasks
-               WHERE company_id = ? AND is_heartbeat = 1 AND status != 'cancelled'
-             ) WHERE rn = 1
-           )`,
-        )
-        .run(ts, companyId, companyId);
-      return Number(result.changes);
-    },
-
     listReviewTasksForManager(managerId: string, companyId: string): TaskRecord[] {
       return readAll<TaskRecord>(
         `select t.id, t.company_id as companyId, t.project_id as projectId, t.goal_id as goalId,
@@ -255,11 +108,11 @@ export function buildTaskStatusMethods(
               t.assignee_agent_id as assigneeAgentId, t.workspace_id as workspaceId,
               t.status, t.priority, t.active_run_id as activeRunId,
               t.session_json as sessionJson, t.session_display_id as sessionDisplayId,
-              t.cost_usd as costUsd, t.is_heartbeat as isHeartbeat,
+              t.cost_usd as costUsd,
               t.created_at as createdAt, t.updated_at as updatedAt
        from tasks t
        inner join agents a on t.assignee_agent_id = a.id
-       where t.company_id = ? and t.status = 'in_review' and a.reports_to = ? and t.is_heartbeat = 0
+       where t.company_id = ? and t.status = 'in_review' and a.reports_to = ?
        order by ${taskPriorityOrderSql("t.priority")} desc, t.created_at asc`,
         companyId,
         managerId,