feat(tests): add Blockbuster runtime gate for event-loop blocking IO (#3229)

* feat(tests): add Blockbuster runtime gate for event-loop blocking IO

Adds a strict runtime gate that fails CI when sync blocking IO calls run
on the asyncio event loop thread through DeerFlow business code.

Components:
- backend/tests/support/detectors/blocking_io_runtime.py — Blockbuster
  context scoped to `app.*` and `deerflow.*` so test infrastructure,
  pytest internals, and third-party libraries stay silent.
- backend/tests/blocking_io/conftest.py — pytest_runtest_protocol
  hookwrapper that wraps every item (setup + call + teardown) with the
  strict context. Respects `@pytest.mark.allow_blocking_io` opt-out.
- backend/tests/blocking_io/test_skills_load.py — regression anchor for
  the #1917 fix (asyncio.to_thread offload around
  LocalSkillStorage.load_skills).
- backend/tests/blocking_io/test_sqlite_lifespan.py — regression anchor
  for the #1912 fix (asyncio.to_thread offload around
  ensure_sqlite_parent_dir).
- backend/tests/blocking_io/test_gate_smoke.py — meta-test asserting the
  gate actually catches unoffloaded blocking IO and that the
  `@pytest.mark.allow_blocking_io` opt-out works.
- backend/Makefile — `make test-blocking-io` target.
- .github/workflows/backend-blocking-io-tests.yml — hard-fail PR gate on
  ubuntu-latest. Windows matrix deferred to follow-up.

Dependencies:
- blockbuster>=1.5.26,<1.6 added to dev group.

Coverage boundary (called out in PR body): the gate only catches blocking
IO on code paths the test suite actually exercises. Static AST inventory
(separate, informational) is the complementary coverage tool. Three blind
spot categories — untested paths, mocked-away paths, env-mismatched paths
— are documented in the PR description.

Findings surfaced while authoring this PR:
- resolve_sqlite_conn_str in runtime/store/_sqlite_utils.py:19 does sync
  Path.resolve() -> os.path.abspath on the lifespan loop thread, ahead of
  the #1912 fix. Not addressed here; tracked as follow-up.

Tests: 4 passed locally (`make test-blocking-io`).
Lint/format: clean (`ruff check` and `ruff format --check`).

* fix(tests): scope Blockbuster gate to blocking-io suite

* fix(tests): harden Blockbuster runtime gate

* test(blocking-io): add project rule extension point

* test(blocking-io): address review cleanup

backend/CLAUDE.md

@@ -88,18 +88,38 @@ make stop       # Stop all services
 
 **Backend directory** (for backend development only):
 ```bash
-make install    # Install backend dependencies
-make dev        # Run Gateway API with reload (port 8001)
-make gateway    # Run Gateway API only (port 8001)
-make test       # Run all backend tests
-make lint       # Lint with ruff
-make format     # Format code with ruff
+make install            # Install backend dependencies
+make dev                # Run Gateway API with reload (port 8001)
+make gateway            # Run Gateway API only (port 8001)
+make test               # Run all backend tests
+make test-blocking-io   # Run strict Blockbuster runtime gate on tests/blocking_io/
+make lint               # Lint with ruff
+make format             # Format code with ruff
 ```
 
 Regression tests related to Docker/provisioner behavior:
 - `tests/test_docker_sandbox_mode_detection.py` (mode detection from `config.yaml`)
 - `tests/test_provisioner_kubeconfig.py` (kubeconfig file/directory handling)
 
+Blocking-IO runtime gate (`tests/blocking_io/`):
+- Wraps every item under `tests/blocking_io/` with a strict Blockbuster
+  context scoped to `app.*` and `deerflow.*` (see
+  `tests/support/detectors/blocking_io_runtime.py`). Any sync blocking IO
+  call whose stack passes through DeerFlow business code while running on
+  the asyncio event loop raises `BlockingError` and fails the test.
+- Two regression anchors live there: `test_skills_load.py` (locks the
+  `asyncio.to_thread` offload around `LocalSkillStorage.load_skills`, fix
+  for #1917) and `test_sqlite_lifespan.py` (locks the offload around
+  SQLite path resolution plus `ensure_sqlite_parent_dir`, fix for #1912).
+- `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.
+- Coverage boundary: the gate only sees code that test execution actually
+  touches. Static AST coverage is a separate concern (out of scope for
+  this PR).
+- CI: runs on every PR via `.github/workflows/backend-blocking-io-tests.yml`,
+  hard-fail.
+
 Boundary check (harness → app import firewall):
 - `tests/test_harness_boundary.py` — ensures `packages/harness/deerflow/` never imports from `app.*`