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/renderer/components/dashboard/Dashboard.tsx

@@ -70,7 +70,7 @@ export function Dashboard(props: DashboardProps) {
       .filter((r) => r.status === "running" || r.status === "queued")
       .map((run) => {
         const agent = agentMap.get(run.agentId);
-        const task = taskMap.get(run.taskId);
+        const task = run.taskId ? taskMap.get(run.taskId) : undefined;
         return { run, agent, task };
       })
       .slice(0, 8);

src/renderer/components/runs/RunCenter.tsx

@@ -51,7 +51,7 @@ export function RunCenter({
       if (!query) {
         return true;
       }
-      const task = taskMap.get(run.taskId);
+      const task = run.taskId ? taskMap.get(run.taskId) : undefined;
       const agent = agentMap.get(run.agentId);
       const workspace = run.workspaceId ? workspaceMap.get(run.workspaceId) : null;
       return [

src/renderer/components/runs/RunDetail.tsx

@@ -47,7 +47,7 @@ export function RunDetail({
   onLoadOlderLogs,
   t,
 }: RunDetailProps) {
-  const task = taskMap.get(selectedRun.taskId);
+  const task = selectedRun.taskId ? taskMap.get(selectedRun.taskId) : undefined;
   const agent = agentMap.get(selectedRun.agentId);
 
   return (

src/renderer/components/runs/RunQueue.tsx

@@ -62,7 +62,7 @@ export function RunQueue({
         </div>
         <ListFrame>
           {filteredRuns.map((run) => {
-            const task = taskMap.get(run.taskId);
+            const task = run.taskId ? taskMap.get(run.taskId) : undefined;
             const agent = agentMap.get(run.agentId);
             const workspace = run.workspaceId ? workspaceMap.get(run.workspaceId) : null;
             const title = task?.title ?? (run.summary || `Run ${run.id.slice(0, 8)}`);