feat(tasks): OpenWorkflow-backed TaskEngine, task registry, outbox relay and op-ledger fencing (W1-4)

- TaskEngine port + OpenWorkflowEngine ([email protected].1, schema openworkflow):
  enqueue/signal/cancel/get/list/startWorker/stop
- defineTask registry with zod input schema; 9 stub task definitions (§3.1-8)
- TaskContext: step.run/sleep/waitForFact, redacting logger, traceId,
  attemptToken, abortSignal driven by PG cancel flag
- signal persists fact to tasks_fact first; waitForFact checks DB before waiting
- OutboxRelay: FOR UPDATE SKIP LOCKED, idempotencyKey outbox:<id>
- withAttemptToken fencing on tasks_op_ledger (own migration, merged in W2)
- tests: kill -9 crash recovery (child worker), cancel, idempotency,
  concurrent relays, early fact, lease-loss fencing

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/tasks/test/fixtures/worker.ts

@@ -0,0 +1,13 @@
+// 崩溃恢复测试用的 worker 子进程:SCHEMA / OW_SCHEMA 指定测试 schema,HANG_STEP3=1 让第 3 步卡住
+import { connectDb, setupEngine } from "../helpers";
+import { createTestRegistry } from "./tasks";
+
+const db = connectDb(process.env.SCHEMA!);
+const engine = await setupEngine(db, { ow: process.env.OW_SCHEMA! }, createTestRegistry(db));
+await engine.startWorker({ versions: ["threeSteps@1"], concurrency: 2 });
+console.log(`worker ready pid=${process.pid}`);
+process.on("SIGTERM", async () => {
+  await engine.stop();
+  await db.destroy();
+  process.exit(0);
+});