chore(blocking-io): fail-loud repo-root resolution and shared detector CLI shim (#3512)

* chore(blocking-io): fail-loud repo-root resolution and shared detector CLI shim

The three detectors resolved REPO_ROOT with depth-indexed
Path(__file__).resolve().parents[4]. If a detector file ever moves to a
different directory depth, scan roots resolve under the wrong directory
and the detector reports zero findings with no error — a silent-zero
failure shape for a detection tool.

- Add support/detectors/repo_root.py: resolve the repo root by walking
  upward to the .git marker (checked with exists() so git worktrees,
  where .git is a file, also resolve), raising RuntimeError when no
  marker is found. All three detectors use it at import time, so a
  relocated detector fails loudly instead of scanning an empty tree.
- Extract scripts/_detector_cli.py from the three character-identical
  CLI shims; the sys.path computation lives in one place and raises
  when backend/tests cannot be found.
- tests/test_detector_repo_root.py pins: resolution from an unmarked
  location raises instead of returning an empty scan; all three
  detectors share the resolved root; each CLI shim delegates to its
  detector.

Testing: backend `make test` (4278 passed); smoke-ran
`make detect-blocking-io`, `make detect-thread-boundaries`, and
`scripts/scan_changed_blocking_io.py --base upstream/main`.

Closes #3510 (review follow-up to #3503).

* chore(blocking-io): declare detector modules import-only, drop script-mode residue

Adversarial review caught that blocking_io_static.py and
thread_boundaries.py kept shebangs and __main__ blocks but can no longer
run as plain scripts: the new `from support.detectors.repo_root import`
executes before anything puts backend/tests on sys.path, so direct
invocation dies with ModuleNotFoundError before argparse.

Direct execution was never a documented entry point (Makefile targets,
the scripts/ shims, the blocking-io-guard skill, and tests all go
through the support.detectors package), so converge on import-only
instead of re-adding per-module bootstrap: drop the shebangs and the now
unreachable __main__ blocks (plus the `import sys` they kept alive) and
state the supported entry points in each module docstring. The shim
delegation tests in test_detector_repo_root.py pin the supported CLI
paths.

Testing: backend `make test` (4278 passed); `make detect-blocking-io`
and `make detect-thread-boundaries` smoke-ran.

backend/tests/test_detector_repo_root.py

@@ -0,0 +1,53 @@
+"""Pin fail-loud repo-root resolution and the shared CLI shim for the detector tooling.
+
+The failure mode being guarded: depth-indexed `parents[N]` resolution from a
+relocated detector file silently resolves a wrong root, scans nothing, and
+reports zero findings. Resolution must instead raise when no repository
+marker is found.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+from support.detectors import blocking_io_changed, blocking_io_static, thread_boundaries
+from support.detectors.repo_root import resolve_repo_root
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+
+
+def test_resolve_repo_root_finds_marker_from_detector_location():
+    resolved = resolve_repo_root(Path(blocking_io_static.__file__))
+    assert resolved == REPO_ROOT
+    assert (resolved / ".git").exists()
+
+
+def test_all_detectors_share_the_resolved_root():
+    assert blocking_io_static.REPO_ROOT == REPO_ROOT
+    assert blocking_io_changed.REPO_ROOT == REPO_ROOT
+    assert thread_boundaries.REPO_ROOT == REPO_ROOT
+
+
+def test_unmarked_location_raises_instead_of_scanning_nothing(tmp_path: Path):
+    start = tmp_path / "moved" / "detectors" / "blocking_io_static.py"
+    with pytest.raises(RuntimeError, match=r"\.git"):
+        resolve_repo_root(start)
+
+
+def test_cli_shims_delegate_to_their_detectors(capsys: pytest.CaptureFixture[str]):
+    # conftest puts scripts/ on sys.path; --help proves the shim resolves and
+    # invokes the right detector main without running a scan.
+    import detect_blocking_io_static
+    import detect_thread_boundaries
+    import scan_changed_blocking_io
+
+    for shim, description_fragment in (
+        (detect_blocking_io_static, "Statically inventory blocking IO calls"),
+        (detect_thread_boundaries, "Detect async/thread boundary points"),
+        (scan_changed_blocking_io, "blocking-IO candidates this change introduces"),
+    ):
+        with pytest.raises(SystemExit) as excinfo:
+            shim.main(["--help"])
+        assert excinfo.value.code == 0
+        assert description_fragment in capsys.readouterr().out