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/snapshot.ts

@@ -75,9 +75,9 @@ export function buildSnapshotMethods(
         ...companyArgs,
       ),
       tasks: readAll<TaskRecord>(
-        `select id, company_id as companyId, project_id as projectId, goal_id as goalId, parent_id as parentId, title, description, assignee_agent_id as assigneeAgentId, workspace_id as workspaceId, status, priority, active_run_id as activeRunId, session_json as sessionJson, session_display_id as sessionDisplayId, cost_usd as costUsd, task_type as taskType, requires_user_review as requiresUserReview, is_heartbeat as isHeartbeat, started_at as startedAt, completed_at as completedAt, cancelled_at as cancelledAt, created_at as createdAt, updated_at as updatedAt from tasks${companyWhere ? `${companyWhere} and is_heartbeat = 0` : " where is_heartbeat = 0"} order by created_at desc limit 2000`,
+        `select id, company_id as companyId, project_id as projectId, goal_id as goalId, parent_id as parentId, title, description, assignee_agent_id as assigneeAgentId, workspace_id as workspaceId, status, priority, active_run_id as activeRunId, session_json as sessionJson, session_display_id as sessionDisplayId, cost_usd as costUsd, task_type as taskType, requires_user_review as requiresUserReview, started_at as startedAt, completed_at as completedAt, cancelled_at as cancelledAt, created_at as createdAt, updated_at as updatedAt from tasks${companyWhere} order by created_at desc limit 2000`,
         ...companyArgs,
-      ).map((record) => ({ ...record, requiresUserReview: toBoolean(record.requiresUserReview), isHeartbeat: toBoolean(record.isHeartbeat), startedAt: record.startedAt ?? null, completedAt: record.completedAt ?? null, cancelledAt: record.cancelledAt ?? null })) as TaskRecord[],
+      ).map((record) => ({ ...record, requiresUserReview: toBoolean(record.requiresUserReview), startedAt: record.startedAt ?? null, completedAt: record.completedAt ?? null, cancelledAt: record.cancelledAt ?? null })) as TaskRecord[],
       approvals: readAll<ApprovalRecord>(
         `select id, company_id as companyId, related_task_id as relatedTaskId, related_agent_id as relatedAgentId, requested_by_agent_id as requestedByAgentId, type, payload_summary as payloadSummary, impact_summary as impactSummary, payload_json as payloadJson, decision_note as decisionNote, state, created_at as createdAt, updated_at as updatedAt from approvals${companyWhere} order by created_at desc limit 1000`,
         ...companyArgs,