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/orchestration/turn-context.ts

@@ -18,7 +18,10 @@ export interface RunDispatchParams {
   runId: string;
   companyId: string;
   agent: ReturnType<AppDatabase["getAgent"]>;
-  task: ReturnType<AppDatabase["getTask"]>;
+  // `task` is null for heartbeat runs (agent wake-ups with no assigned task).
+  // Downstream code (prompt builder, worker payload, session pointer)
+  // branches on this — when null, skip task-scoped state.
+  task: ReturnType<AppDatabase["getTask"]> | null;
   connector: ReturnType<AppDatabase["getConnector"]>;
   workspace: ReturnType<AppDatabase["getWorkspace"]>;
   workspaceMode: WorkspaceLockMode;
@@ -145,6 +148,15 @@ function resolveSession(
   // that read per-task active_session_id keep working.
   db.updateTasksActiveSessionForAgent(agent.id, agentSessionId);
 
+  // Heartbeat runs have no task; reuse the agent-scoped session with no
+  // task-specific active session lookup.
+  if (!task) {
+    return {
+      taskSession: { id: agentSessionId, sessionParams: null, sessionDisplayId: null },
+      sessionHandoff,
+    };
+  }
+
   const activeSession = db.getActiveTaskSession(task.id);
   let taskSession: {
     id: string | null;
@@ -267,12 +279,12 @@ export function buildRunPayload(params: RunDispatchParams, deps: TurnContextDeps
   const { db } = deps;
 
   const { env, apiUrl, apiKey } = assembleApiEnv(db, connector, companyId, agent.id, runId, trigger, deps);
-  env.AGENT_COMPANY_TASK_ID = task.id;
+  env.AGENT_COMPANY_TASK_ID = task?.id ?? "";
 
   const assignedTasks = db.listAgentTasks(agent.id, companyId, ["todo", "in_progress", "blocked"]);
   const reviewTasks = db.listReviewTasksForManager(agent.id, companyId);
   const recentComments = assembleRecentComments(assignedTasks, reviewTasks, snapshot);
-  const initialHandoff = resolveSessionHandoff(task.id, deps);
+  const initialHandoff = task ? resolveSessionHandoff(task.id, deps) : null;
   const { taskSession, sessionHandoff } = resolveSession(db, agent, task, companyId, connector.id, initialHandoff, deps);
   const skills = resolveSkills(companyId, agent.id, connector.id, agent.name, deps);
 
@@ -331,9 +343,9 @@ export function buildRunPayload(params: RunDispatchParams, deps: TurnContextDeps
   return {
     runId,
     companyId,
-    taskId: task.id,
-    taskTitle: task.title,
-    taskDescription: task.description,
+    taskId: task?.id ?? null,
+    taskTitle: task?.title ?? "",
+    taskDescription: task?.description ?? "",
     agentId: agent.id,
     agentName: agent.name,
     connectorId: connector.id,