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/package.json

@@ -9,5 +9,16 @@
   "scripts": {
     "typecheck": "tsc -p tsconfig.json",
     "test": "vitest run"
+  },
+  "dependencies": {
+    "@umt/db": "workspace:*",
+    "kysely": "^0.29.6",
+    "openworkflow": "0.10.1",
+    "pg": "^8.23.0",
+    "postgres": "^3.4.9",
+    "zod": "^4.6.5"
+  },
+  "devDependencies": {
+    "@types/pg": "^8.23.1"
   }
 }