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-base.ts

@@ -268,14 +268,11 @@ export class AppDatabaseBase {
   checkoutTask(taskId: string, runId: string) { return this.tasks.checkoutTask(taskId, runId); }
   releaseTaskLock(taskId: string, runId: string) { return this.tasks.releaseTaskLock(taskId, runId); }
   renewTaskLease(taskId: string, runId: string) { return this.tasks.renewTaskLease(taskId, runId); }
-  listAgentTasks(agentId: string, companyId: string, statuses: string[], options?: { includeHeartbeat?: boolean }) { return this.tasks.listAgentTasks(agentId, companyId, statuses, options); }
-  listTasksByStatuses(companyId: string, statuses: string[], options?: { includeHeartbeat?: boolean }) { return this.tasks.listTasksByStatuses(companyId, statuses, options); }
+  listAgentTasks(agentId: string, companyId: string, statuses: string[]) { return this.tasks.listAgentTasks(agentId, companyId, statuses); }
+  listTasksByStatuses(companyId: string, statuses: string[]) { return this.tasks.listTasksByStatuses(companyId, statuses); }
   listTasksForStandup(companyId: string) { return this.tasks.listTasksForStandup(companyId); }
   listAssigneeTasksForStandup(companyId: string, agentId: string) { return this.tasks.listAssigneeTasksForStandup(companyId, agentId); }
   listTasksForAssignees(companyId: string, assigneeIds: string[]) { return this.tasks.listTasksForAssignees(companyId, assigneeIds); }
-  findReusableHeartbeatTask(agentId: string, companyId: string) { return this.tasks.findReusableHeartbeatTask(agentId, companyId); }
-  acquireHeartbeatTask(input: { agentId: string; companyId: string; runId: string; workspaceId: string | null; title: string; description: string; priority: string }) { return this.tasks.acquireHeartbeatTask(input); }
-  cleanupDuplicateHeartbeatTasks(companyId: string) { return this.tasks.cleanupDuplicateHeartbeatTasks(companyId); }
   listReviewTasksForManager(managerId: string, companyId: string) { return this.tasks.listReviewTasksForManager(managerId, companyId); }
   countActiveTasksForCompany(companyId: string) { return this.tasks.countActiveTasksForCompany(companyId); }
   listChildTasks(parentId: string, companyId: string) { return this.tasks.listChildTasks(parentId, companyId); }
@@ -300,7 +297,7 @@ export class AppDatabaseBase {
 
   getRun(id: string) { return this.runs.getRun(id); }
   getRunAuthState(runId: string) { return this.runs.getRunAuthState(runId); }
-  createRun(input: { id?: string; companyId: string; taskId: string; agentId: string; workspaceId: string | null; connectorId: ConnectorId }) { return this.runs.createRun(input); }
+  createRun(input: { id?: string; companyId: string; taskId: string | null; agentId: string; workspaceId: string | null; connectorId: ConnectorId }) { return this.runs.createRun(input); }
   appendRunLog(runId: string, stream: "stdout" | "stderr", chunk: string) { return this.runs.appendRunLog(runId, stream, chunk); }
   readRunLog(runId: string) { return this.runs.readRunLog(runId); }
   readRunLogChunk(runId: string, offset = 0, limit = 65_536) { return this.runs.readRunLogChunk(runId, offset, limit); }