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/src/errors.ts

@@ -0,0 +1,21 @@
+export class TaskCanceledError extends Error {
+  override name = "TaskCanceledError";
+  constructor(runId: string, reason?: string | null) {
+    super(`task run ${runId} canceled${reason ? `: ${reason}` : ""}`);
+  }
+}
+
+/** 租约已丢:该 opKey 已有更新的 attempt,当前 attempt 的提交被拒绝 */
+export class FencingError extends Error {
+  override name = "FencingError";
+  constructor(opKey: string) {
+    super(`operation ${opKey}: attempt token superseded, commit rejected`);
+  }
+}
+
+export class UnknownTaskError extends Error {
+  override name = "UnknownTaskError";
+  constructor(name: string, version?: number) {
+    super(`task ${name}${version === undefined ? "" : `@${version}`} is not registered`);
+  }
+}