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).

tests/unit/task-quality-gates.test.ts

@@ -11,14 +11,14 @@ import type { TaskValidationResult as _TaskValidationResult } from "../../src/ma
 
 describe("validateTaskCreation", () => {
   it("rejects description with 5 chars when not heartbeat", () => {
-    const result = validateTaskCreation({ title: "Fix bug", description: "Short" }, false);
+    const result = validateTaskCreation({ title: "Fix bug", description: "Short" });
     expect(result.valid).toBe(false);
     expect(result.errors.some(e => e.includes("at least 50 characters"))).toBe(true);
   });
 
   it("rejects description with exactly 49 chars when not heartbeat", () => {
     const desc = "A".repeat(49);
-    const result = validateTaskCreation({ title: "Fix bug", description: desc }, false);
+    const result = validateTaskCreation({ title: "Fix bug", description: desc });
     expect(result.valid).toBe(false);
     expect(result.errors.some(e => e.includes("at least 50 characters"))).toBe(true);
   });
@@ -29,20 +29,13 @@ describe("validateTaskCreation", () => {
     const padded = desc.slice(0, 50).padEnd(50, ".");
     expect(padded.length).toBe(50);
     // Must contain "done when"
-    const result = validateTaskCreation({ title: "Build login page", description: padded }, false);
+    const result = validateTaskCreation({ title: "Build login page", description: padded });
     expect(result.valid).toBe(true);
     expect(result.errors).toHaveLength(0);
   });
 
-  it("always returns valid for heartbeat tasks regardless of description", () => {
-    const result = validateTaskCreation({ title: "[Heartbeat] Check status", description: "Short" }, true);
-    expect(result.valid).toBe(true);
-    expect(result.errors).toHaveLength(0);
-    expect(result.warnings).toHaveLength(0);
-  });
-
   it("rejects empty description when not heartbeat", () => {
-    const result = validateTaskCreation({ title: "Some task", description: "" }, false);
+    const result = validateTaskCreation({ title: "Some task", description: "" });
     expect(result.valid).toBe(false);
     expect(result.errors.some(e => e.includes("at least 50 characters"))).toBe(true);
   });
@@ -50,7 +43,7 @@ describe("validateTaskCreation", () => {
   it("rejects 60-char description without acceptance criteria keywords", () => {
     const desc = "This is a description that is long enough to pass length check";
     expect(desc.length).toBeGreaterThanOrEqual(50);
-    const result = validateTaskCreation({ title: "A task", description: desc }, false);
+    const result = validateTaskCreation({ title: "A task", description: desc });
     expect(result.valid).toBe(false);
     expect(result.errors.some(e => e.toLowerCase().includes("acceptance criteria"))).toBe(true);
   });
@@ -58,13 +51,13 @@ describe("validateTaskCreation", () => {
   it("returns valid=true when description has both length and acceptance criteria", () => {
     const desc = "Implement the authentication flow for the login page. Done when: all tests pass and the form submits.";
     expect(desc.length).toBeGreaterThanOrEqual(50);
-    const result = validateTaskCreation({ title: "Auth flow", description: desc }, false);
+    const result = validateTaskCreation({ title: "Auth flow", description: desc });
     expect(result.valid).toBe(true);
     expect(result.errors).toHaveLength(0);
   });
 
   it("includes current character count in length error message", () => {
-    const result = validateTaskCreation({ title: "Fix", description: "Short" }, false);
+    const result = validateTaskCreation({ title: "Fix", description: "Short" });
     expect(result.errors.some(e => e.includes("currently 5"))).toBe(true);
   });
 });