fix(agents): offload UploadsMiddleware uploads scan off the event loop (#3311)

UploadsMiddleware defines only the sync `before_agent` hook. LangChain wires a
sync-only hook as `RunnableCallable(before_agent, None)`, and LangGraph's
`ainvoke` runs it directly on the event loop when `afunc is None` — so the
per-message uploads-directory scan (`exists`/`iterdir`/`stat` plus reading
sibling `.md` outlines) blocks the asyncio event loop on every message that has
an uploads directory.

Add `abefore_agent` that offloads the scan to a worker thread via
`run_in_executor`; it copies the current context, preserving the `user_id`
contextvar read by `get_effective_user_id()`.

Add a runtime anchor under `tests/blocking_io/` that drives the real
`create_agent` graph via `ainvoke` under the strict Blockbuster gate, so a
regression back onto the event loop fails CI. Update blocking-IO docs.

backend/CLAUDE.md

@@ -126,8 +126,10 @@ Blocking-IO runtime gate (`tests/blocking_io/`):
   `asyncio.to_thread` offload around `LocalSkillStorage.load_skills`, fix
   for #1917); `test_sqlite_lifespan.py` (locks the offload around
   SQLite path resolution plus `ensure_sqlite_parent_dir`, fix for #1912);
-  and `test_jsonl_run_event_store.py` (locks `JsonlRunEventStore`'s async
-  API offloading its file IO via `asyncio.to_thread`, fix #3084).
+  `test_jsonl_run_event_store.py` (locks `JsonlRunEventStore`'s async
+  API offloading its file IO via `asyncio.to_thread`, fix #3084); and
+  `test_uploads_middleware.py` (locks `UploadsMiddleware.abefore_agent`
+  offloading the uploads-directory scan off the event loop).
 - `test_gate_smoke.py` is a meta-test asserting the gate actually catches
   unoffloaded blocking IO and that the `@pytest.mark.allow_blocking_io`
   opt-out works.