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/automation/automation-actions.ts

@@ -115,9 +115,7 @@ export function executeAutomationAction(
       const description = cfg.description ?? "";
       const priority = cfg.priority ?? "medium";
       const assigneeAgentId = cfg.assigneeAgentId ?? null;
-      // Automation rules create user-facing tasks; heartbeat tasks are
-      // synthesized only by acquireHeartbeatTask, never via create_task.
-      const taskValidation = validateTaskCreation({ title, description }, false);
+      const taskValidation = validateTaskCreation({ title, description });
       if (!taskValidation.valid) {
         logger.warn(`[automation] Skipping create_task — validation failed: ${taskValidation.errors.join("; ")}`);
         break;
@@ -231,20 +229,6 @@ export function executeAutomationAction(
       } catch (err) {
         logger.warn(`[automation] getTask failed for escalation task ${ctx.taskId}; using id as title:`, err instanceof Error ? err.message : String(err));
       }
-      // Heartbeat tasks are internal system tasks — skip escalation messages to avoid flooding channels.
-      let isHeartbeatTask = false;
-      if (ctx.taskId) {
-        try {
-          isHeartbeatTask = db.getTask(ctx.taskId).isHeartbeat === true;
-        } catch (err) {
-          logger.warn(`[automation] getTask failed for heartbeat check on ${ctx.taskId}; treating as non-heartbeat:`, err instanceof Error ? err.message : String(err));
-          isHeartbeatTask = false;
-        }
-      }
-      if (isHeartbeatTask) {
-        logger.info(`[automation] Skipping escalation for heartbeat task "${taskTitle}"`);
-        break;
-      }
       const manager = db.getManagerForAgent(targetAgentId);
       const escalationBody = (config.body as string) ?? `Task "${taskTitle}" needs your attention. ${agentName} has flagged this as requiring manager intervention.`;
       const wakeTrigger = deriveEscalationWakeTrigger(ctx.newStatus ?? null, rule.trigger ?? null);

src/main/automation/automation-run-completion.ts

@@ -209,6 +209,7 @@ export function processRunCompletion(
   const { db, logger: _logger, publishDomainChanged, notify, wakeAgentIfPossible } = deps;
   const { dispatchAutomation, handleTaskStatusChange } = callbacks;
 
+  if (!run.taskId) return;
   let task: ReturnType<typeof db.getTask> | null = null;
   try {
     task = db.getTask(run.taskId);
@@ -260,14 +261,9 @@ export function processRunCompletion(
     }
   }
 
-  if (isSuccess && task.isHeartbeat && effectiveStatus !== "done") {
-    db.advanceTaskStatus(task.id, "done", "Heartbeat check completed.");
-    handleTaskStatusChange(task.id, run.companyId, effectiveStatus, "done");
-  }
-
   const automationContext: AutomationEventContext = {
     companyId: run.companyId,
-    taskId: run.taskId,
+    taskId: run.taskId ?? undefined,
     agentId: run.agentId,
     runId: run.id,
   };