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/task-runner.ts

@@ -7,7 +7,7 @@ import type { RunDispatchParams } from "./turn-context";
 
 export function createTaskRunner(
   deps: OrchestrationDependencies,
-  resolveWorkspace: (task: { workspaceId: string | null; projectId?: string | null }, agent: { workspaceId: string | null }, companyId: string) => { workspace: ReturnType<AppDatabase["getWorkspace"]>; lockMode: "exclusive" | "shared" | "none"; source: string },
+  resolveWorkspace: (task: { workspaceId: string | null; projectId?: string | null } | null, agent: { workspaceId: string | null }, companyId: string) => { workspace: ReturnType<AppDatabase["getWorkspace"]>; lockMode: "exclusive" | "shared" | "none"; source: string },
   getRunEligibilityFailure: (agent: AgentRecord, connector: ConnectorRecord, projectId?: string | null) => string | null,
   dispatchRunToWorker: (params: RunDispatchParams) => void,
   hasActiveRun: (filter: { taskId?: string; agentId?: string; companyId: string }) => boolean,
@@ -167,10 +167,10 @@ export function createTaskRunner(
       }
 
       try {
-        const task = db.getTask(run.taskId);
+        const task = run.taskId ? db.getTask(run.taskId) : null;
         const agent = db.getAgent(run.agentId);
         const connector = db.getConnector(run.connectorId);
-        const eligibilityFailure = getRunEligibilityFailure(agent, connector, task.projectId ?? null);
+        const eligibilityFailure = getRunEligibilityFailure(agent, connector, task?.projectId ?? null);
         if (eligibilityFailure) {
           if (eligibilityFailure === "CONNECTOR_MODEL_NOT_ALLOWED") {
             db.finishRun({
@@ -190,13 +190,13 @@ export function createTaskRunner(
               action: "run.recovery_blocked",
               entityType: "run",
               entityId: run.id,
-              detail: `Recovered queued run blocked by connector/model policy for task "${task.title}"`,
+              detail: `Recovered queued run blocked by connector/model policy for task "${task?.title ?? "(heartbeat)"}"`,
             });
             emitEvent({ type: "run-updated", runId: run.id, action: "completed" });
           }
           continue;
         }
-        const workspaceResolution = resolveWorkspace(task, agent, run.companyId);
+        const workspaceResolution = resolveWorkspace(task ?? null, agent, run.companyId);
         dispatchRunToWorker({
           runId: run.id,
           companyId: run.companyId,