feat(skill): add blocking-io-guard — SOP skill for blocking-IO triage and runtime anchors (#3503)

* feat(blocking-io): add changed-lines blocking-IO scanner (L1)

* feat(blocking-io): add scan-changed CLI wrapper

* feat(skill): add blocking-io-guard developer SOP skill

* docs(blocking-io): point contributors at the blocking-io-guard skill

* style(blocking-io): apply ruff format to scanner and tests

* docs(backend): document changed-lines blocking-IO scanner in CLAUDE.md

* feat(skill): add post-fix re-scan check and PR batching policy

* refactor(skill): fix SOP step ordering, align template with repo conventions

- Move re-scan into an explicit 'apply the fix' step (was wedged after
  anchor generation while telling you to go back before the anchor)
- Renumber steps 0-6; drop undefined 'L1' jargon
- Mode A: document that the diff is <base>...HEAD (commit first)
- Mode B: prefer make detect-blocking-io + findings JSON file
- anchor template: module-level pytestmark per tests/blocking_io convention
- CLAUDE.md: fix 'git diff --base' phrasing

* fix(skill): catch findings introduced without touching the blocking line

Review follow-up: changed-line intersection alone misses the case where a
new async caller exposes an old sync helper — the static finding sits on
the untouched blocking line, so Mode A returned empty and the SOP stopped
on a false 'no blocking-IO surface'.

Selection is now a union over the changed files:
- findings on added lines of git diff <base>...HEAD (kept: a second
  identical symbol in an already-flagged function collides on the stable
  key and only this selection sees it);
- findings new versus the merge base, matched by (path, function,
  symbol) — never line numbers.

Base sources are materialized via git show <merge-base>:<path>; files
absent at base count every head finding as new. SKILL.md now states the
residual same-file-only blind spot (cross-file async callers) instead of
treating an empty list as proof of zero exposure, and only requires
reading sop-skeleton.md when generalizing to another detector domain.

* docs(skill): examples teach test-writing, the teeth check defines the rule

All examples in the references/template are filesystem-flavored; make
explicit that they are instances, not the SOP's boundary — the same rules
apply to every detector category (FILE_IO, HTTP, SUBPROCESS, SLEEP) and
acceptance is always red/green teeth, never similarity to an example.
Neutralize the template's arrange comment accordingly.

* fix(blocking-io): harden changed-lines scanner per review

- Dedup the union selection by the stable key (path, function, symbol)
  instead of dict identity, so a future selector returning copied dicts
  cannot silently empty the result.
- parse_changed_lines now handles any unified diff: context lines advance
  the new-file counter, \-markers and deletions do not, and the counter
  resets at each +++ header. Previously correct only for --unified=0.
- Add blocking_io_static.scan_source (in-memory scan); base-version
  comparison no longer round-trips through temp files.
- Empty Mode A report now prints the same-file-only reachability caveat
  at the point of use instead of relying on the SOP text alone.

* docs(skill): bound best-effort cleanup when the offload sits in finally

Lesson from the #3505 review: the SOP routinely drives 'offload the
cleanup branch' transformations, and an awaited cleanup in finally can
mask or stall the primary exception. One sentence in Step 2 closes that
gap at the point where the fix is written.

scripts/scan_changed_blocking_io.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+"""CLI wrapper for the changed-lines blocking IO scanner."""
+
+from __future__ import annotations
+
+import sys
+from collections.abc import Sequence
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+TEST_SUPPORT_PATH = REPO_ROOT / "backend" / "tests"
+if str(TEST_SUPPORT_PATH) not in sys.path:
+    sys.path.insert(0, str(TEST_SUPPORT_PATH))
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    from support.detectors.blocking_io_changed import main as scanner_main
+
+    return scanner_main(argv)
+
+
+if __name__ == "__main__":
+    sys.exit(main())