fix: CI trust-boundary + fail-closed repairs (adversarial findings)

- Token/exec separation restored: slices-report (runs PR-authored code:
  bun install + the reconcile runner) drops to contents:read; the PR
  comment moves to a NEW slices-comment job holding the write token with
  ZERO repo code — no checkout, no bun, only downloaded artifacts + jq/gh.
  $GITHUB_ENV/BASH_ENV persistence is job-scoped, so the split is the
  boundary. The matrix-era report job had this property; the consolidation
  had regressed it. Pinned by the wiring test.
- Reconcile exit captured via PIPESTATUS[0] in BOTH lanes: GitHub's default
  run-step shell has no pipefail, so `$?` after `| tee` was tee's exit —
  the fail-closed gate was silently fail-open. Wiring test pins it.
- PR comment: final-attempt accounting restored the dropped COST
  accumulation (the dial read $0 forever), flaky passes render as the
  warning they are (never as failures), and a malformed tests[] artifact
  skips that file instead of aborting the whole comment under bash -e.
- Remaining mutable action tags pinned (free-tests upload-artifact,
  ci-image checkout/docker trio — the image publisher holds packages:write
  and feeds the secret-bearing lanes). restore-deps fallback installs
  --frozen-lockfile; register-gstack-skills validates skill names before
  its rm -rf.

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

.github/workflows/evals-periodic.yml

@@ -292,7 +292,11 @@ jobs:
         run: |
           set +e
           EVALS_TIER=periodic bun run scripts/test-paid-shards.ts --tier periodic --report /tmp/paid-report | tee /tmp/report.txt
-          echo "exit=$?" >> "$GITHUB_OUTPUT"
+          # PIPESTATUS[0], NOT $?: GitHub's default run-step shell is
+          # `bash -e {0}` with NO pipefail, so $? after the pipe is tee's
+          # exit (always 0) — the fail-closed gate was silently fail-open
+          # (caught by the ship review army; the wiring test now pins this).
+          echo "exit=${PIPESTATUS[0]}" >> "$GITHUB_OUTPUT"
 
       # A red weekly lane nobody must action is waste — upsert ONE tracking
       # issue (never a new issue per week) with the reconciliation output, so