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.

backend/tests/support/detectors/blocking_io_static.py

@@ -717,12 +717,11 @@ def _finalize_findings(visitor: BlockingIOStaticVisitor) -> list[BlockingIOStati
     return findings
 
 
-def scan_file(path: Path, *, repo_root: Path = REPO_ROOT) -> list[BlockingIOStaticFinding]:
-    source = path.read_text(encoding="utf-8")
+def scan_source(source: str, relative_path: str) -> list[BlockingIOStaticFinding]:
+    """Scan one in-memory Python source; `relative_path` is reported verbatim in findings."""
     source_lines = source.splitlines()
-    relative_path = relative_to_repo(path, repo_root)
     try:
-        tree = ast.parse(source, filename=str(path))
+        tree = ast.parse(source, filename=relative_path)
     except SyntaxError as exc:
         line = exc.lineno or 0
         code = _source_snippet(source_lines, line)
@@ -746,6 +745,10 @@ def scan_file(path: Path, *, repo_root: Path = REPO_ROOT) -> list[BlockingIOStat
     return sorted(_finalize_findings(visitor), key=lambda finding: (finding.path, finding.line, finding.column, finding.category))
 
 
+def scan_file(path: Path, *, repo_root: Path = REPO_ROOT) -> list[BlockingIOStaticFinding]:
+    return scan_source(path.read_text(encoding="utf-8"), relative_to_repo(path, repo_root))
+
+
 def is_ignored_path(path: Path) -> bool:
     return any(part in IGNORED_DIR_NAMES for part in path.parts)