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>

.changeset/lively-quails-forage.md

@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 0
+---
+**`/gsd-code-review --files` no longer silently widens back to the whole phase** — Tier 3's SUMMARY/diff cross-check ran regardless of an explicit `--files` override, appending the rest of the phase's changed files onto a scope the user had deliberately narrowed.