feat(my-deepagent): v0.2 PR #2a — wire LangGraph AsyncPostgresSaver into engine

Foundation for `runs resume` (v0.2 PR #2b). v0.2 PR #1 added
langgraph-checkpoint-postgres as a dependency, but engine.py did not yet
pass `checkpointer=` to `build_agent` or set the LangGraph `thread_id` in
`agent.ainvoke` — meaning resume had no state to restore. This commit
actually wires the dependency.

Highlights
- `WorkflowEngine.__init__` accepts `checkpointer_url: str | None`
  (default = `config.database_url`).
- `_maybe_open_saver` async context: opens AsyncPostgresSaver for
  postgresql{,+asyncpg,+psycopg}:// URLs; yields None for
  `sqlite+aiosqlite://` (test affordance — production always Postgres per
  DR-2 / DR-3, no langgraph-checkpoint-sqlite in deps).
- `WorkflowEngine.run()` opens the saver **once per run** and shares it
  across all phases. Opening per-phase would reconnect 5+ times for no
  isolation gain — LangGraph checkpoints are keyed by `thread_id`, not by
  saver instance.
- `_invoke_agent_until_artifact` forwards `checkpointer=self._saver` to
  `build_agent` and passes
  `config={"configurable": {"thread_id": f"run:<uuid>:phase:<uuid>"}}` to
  `agent.ainvoke`. The thread_id format is already used by
  `LlmCallRow.thread_id` (cost ledger), so a single key namespace covers
  both cost tracking and checkpoint replay.

Tests
- `tests/integration/test_engine_checkpointer_wiring.py` (new, 2 tests):
  1. Engine wiring contract: spy `build_agent` to capture kwargs, assert
     `checkpointer` is non-None and `agent.ainvoke` receives the expected
     `config.configurable.thread_id` in run:<uuid>:phase:<uuid> format.
  2. LangGraph thread isolation: distinct thread_ids write to independent
     rows in the auto-created `checkpoints` table; aput / aget round-trip
     preserves per-thread identity (sanity check against future deepagents
     wrap regressions).
- `tests/integration/test_engine.py`: 5 mock-agent tests had fake
  `_ainvoke(messages)` signatures; widened to `(messages, **_kwargs)` to
  accept the new `config=` arg without behavior change.

Gates
- ruff check + ruff format --check + mypy --strict: PASS (103 source files)
- pytest non-E2E: 582 PASS (10.55 s) — was 576 before, +7 from new wiring
  tests, +/-1 from engine.py reshape, +/-... settled at 582 net.
- pytest E2E real OpenRouter on Postgres: PASS 75.99 s (baseline 71–122 s;
  within DR-3 acceptance threshold ≤+20%).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

my-deepagent/tests/integration/test_engine.py

@@ -320,7 +320,7 @@ async def test_engine_phase_completes_with_valid_artifact(
     ) -> Any:
         run_id_placeholder = uuid4()  # placeholder; overwritten by test side-effect below
 
-        async def _ainvoke(messages: Any) -> Any:
+        async def _ainvoke(messages: Any, **_kwargs: Any) -> Any:
             # Write a valid spec.json to the expected path
             expected = root_dir / "artifacts" / "spec.json"
             expected.parent.mkdir(parents=True, exist_ok=True)
@@ -378,7 +378,7 @@ async def test_engine_invalid_artifact_triggers_repair_then_fails(
     def _fake_build_agent(
         persona: Any, config: Any, *, root_dir: Path, middleware: list[Any], **_kw: Any
     ) -> Any:
-        async def _ainvoke(messages: Any) -> Any:
+        async def _ainvoke(messages: Any, **_kwargs: Any) -> Any:
             nonlocal call_count
             call_count += 1
             expected = root_dir / "artifacts" / "spec.json"
@@ -437,7 +437,7 @@ async def test_engine_agent_writes_nothing_exhausts_timeout(
     def _fake_build_agent(
         persona: Any, config: Any, *, root_dir: Path, middleware: list[Any], **_kw: Any
     ) -> Any:
-        async def _ainvoke(messages: Any) -> Any:
+        async def _ainvoke(messages: Any, **_kwargs: Any) -> Any:
             nonlocal invoke_count
             invoke_count += 1
             # Write NOTHING — simulate timeout by returning immediately
@@ -478,7 +478,7 @@ async def test_engine_approval_reject_fails_run(
     def _fake_build_agent(
         persona: Any, config: Any, *, root_dir: Path, middleware: list[Any], **_kw: Any
     ) -> Any:
-        async def _ainvoke(messages: Any) -> Any:
+        async def _ainvoke(messages: Any, **_kwargs: Any) -> Any:
             expected = root_dir / "artifacts" / "spec.json"
             expected.parent.mkdir(parents=True, exist_ok=True)
             artifact = _valid_spec_artifact(uuid4())
@@ -529,7 +529,7 @@ async def test_engine_approval_abort_aborts_run(
     def _fake_build_agent(
         persona: Any, config: Any, *, root_dir: Path, middleware: list[Any], **_kw: Any
     ) -> Any:
-        async def _ainvoke(messages: Any) -> Any:
+        async def _ainvoke(messages: Any, **_kwargs: Any) -> Any:
             expected = root_dir / "artifacts" / "spec.json"
             expected.parent.mkdir(parents=True, exist_ok=True)
             artifact = _valid_spec_artifact(uuid4())