* 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.
AochenShen99