fix(#4460): restore compact-file reachability + fix test stdout capture

Two more gsd-test-surfaced findings:

1. The previous execute-plan.md trim removed the literal
   `summary.compact.md` filename mention, breaking
   tests/compact-content-variant-guard.test.cjs's reachability check
   (ADR-4139 Phase 6): every registered .compact.md variant must be
   named by at least one workflow "spine" file, and execute-plan.md was
   apparently the only spine naming this one. Restored the bare
   filename (kept the shortened surrounding wording) -- read
   tests/helpers/compact-content-variant.cjs's checkReachability/
   isUnprefixedMatch directly to confirm the fix rather than guessing.
   40926 bytes, still 34 under the size cap.

2. The redesigned test (previous commit) still failed: both tiers'
   diagnostic `echo`/`printf "Warning: ..."` lines were mixing into the
   captured stdout the assertions parse as the file list, so
   "--files=src/alpha.js" appeared to produce 2 lines instead of 1.
   Wrapped both tier fences in a `{ ...; } > /dev/null` brace group
   (not a subshell -- REVIEW_FILES still persists to the enclosing
   shell) so only the final printf reaches stdout. Manually re-verified
   both cases against a real git fixture before re-running the suite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/code-review-tier3-files-override-scoping.test.cjs

@@ -117,6 +117,12 @@ function runTiers(tmpDir, { filesOverride, seedReviewFiles = [] }) {
     'set -uo pipefail',
     `FILES_OVERRIDE="${filesOverride || ''}"`,
     filesArrayInit,
+    // Both tiers print diagnostic "File scope: ..." / "Warning: ..." lines to
+    // stdout as documentation for a human running code-review.md interactively
+    // — a brace group (not a subshell: variables set inside still persist to
+    // the enclosing shell) discards that chatter so only the final REVIEW_FILES
+    // printf below reaches this script's captured stdout.
+    '{',
     tier1,
     // Tier 1 unconditionally resets REVIEW_FILES=() when FILES_OVERRIDE is
     // set; the seed only matters (and only applies) when it is not, exactly
@@ -126,6 +132,7 @@ function runTiers(tmpDir, { filesOverride, seedReviewFiles = [] }) {
     'PADDED_PHASE="03"',
     'LAST_REVIEW_COMMIT=""',
     tier3,
+    '} > /dev/null',
     'printf \'%s\\n\' "${REVIEW_FILES[@]}"',
   ].join('\n');