fix(#4460): gate Tier 3's #2666 cross-check on FILES_OVERRIDE

code-review.md states (line 144) "Skip SUMMARY/git scoping entirely when
--files is provided." Tier 2 honors this via `if [ -z "$FILES_OVERRIDE"
]`, but Tier 3's #2666 SUMMARY/diff cross-check had no FILES_OVERRIDE
reference at all -- reached via `elif [ -n "$DIFF_BASE" ]` whenever
REVIEW_FILES was already non-empty (true under --files, since Tier 1
fills it), so it silently appended the whole phase's changed files onto
an explicit user-supplied file list. --files is documented as the
highest-precedence scoping tier (D-08) and is the flag Tier 3's own
fail-closed path recommends when no reliable diff base is found; a user
narrowing a review to two files silently got the whole phase instead,
and the reviewer agent spent its budget on files nobody asked about.

Gated the elif on the same condition Tier 2 already uses:

  elif [ -z "$FILES_OVERRIDE" ] && [ -n "$DIFF_BASE" ]; then

The issue's own narrowest suggested form, reasoned through against two
alternatives (wrapping the whole Tier-3 fence, or changing the stated
invariant instead) -- both explicitly rejected there for good reasons
concurred with after reading the surrounding code.

Added tests/code-review-tier3-files-override-scoping.test.cjs, mirroring
the issue's own verified reproduction methodology: extracts the Tier
1/2/3 fences VERBATIM from code-review.md (never reimplemented) and runs
them against a real constructed git fixture matching the issue's own
scenario exactly (5 files, a SUMMARY listing only 1). Confirms --files
stays scoped to exactly the requested file, and separately confirms the
#2666 cross-check still widens a genuinely partial SUMMARY scope when
--files is absent (proving this is a gate, not a blanket disable).

Emitted-Drift-Ack-Growth: code-review.md — #4460 gates the Tier-3 #2666 cross-check on FILES_OVERRIDE, matching Tier 2's own guard, net +453 bytes
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

gsd-core/workflows/code-review.md

@@ -292,7 +292,13 @@ if [ ${#REVIEW_FILES[@]} -eq 0 ]; then
     echo "Warning: No phase commits found for '${PADDED_PHASE}'. Cannot determine reliable diff scope."
     echo "Use --files flag to specify files explicitly: /gsd:code-review ${PHASE_ARG} --files=file1,file2,..."
   fi
-elif [ -n "$DIFF_BASE" ]; then
+elif [ -z "$FILES_OVERRIDE" ] && [ -n "$DIFF_BASE" ]; then
+  # #4460: gated on FILES_OVERRIDE being unset — without this, REVIEW_FILES is
+  # already non-empty under --files (Tier 1 filled it), so this elif was
+  # reached anyway and the #2666 cross-check below appended the whole phase
+  # diff onto an explicit user-supplied file list, contradicting line 144's
+  # "Skip SUMMARY/git scoping entirely when --files is provided" and Tier 2's
+  # own --files guard immediately above.
   # #2666 cross-check: SUMMARY yielded a non-empty (possibly partial) scope.
   # Warn about — and add — any changed files the SUMMARY extractor did not surface,
   # so a partial result can no longer silently ship an incomplete review scope.