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/ci-image.yml

@@ -36,7 +36,7 @@ jobs:
       contents: read
       packages: write
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
 
       # Copy lockfile + package.json into Docker build context
       - run: cp package.json bun.lock .github/docker/ && cp -R patches .github/docker/patches
@@ -48,7 +48,7 @@ jobs:
       - id: meta
         run: echo "tag=ghcr.io/${{ github.repository }}/ci:${{ hashFiles('.github/docker/Dockerfile.ci', 'bun.lock', 'patches/**') }}" >> "$GITHUB_OUTPUT"
 
-      - uses: docker/login-action@v4
+      - uses: docker/login-action@dbcb813823bdd20940b903addbd779551569679f # v4
         with:
           registry: ghcr.io
           username: ${{ github.actor }}
@@ -69,10 +69,10 @@ jobs:
       # Registry cache export needs a docker-container builder — the default
       # `docker` driver hard-errors on cache-to.
       - if: steps.check.outputs.exists == 'false'
-        uses: docker/setup-buildx-action@v4
+        uses: docker/setup-buildx-action@37fe631027851001ddb9b187196cc803df7f5f0e # v4
 
       - if: steps.check.outputs.exists == 'false'
-        uses: docker/build-push-action@v7
+        uses: docker/build-push-action@53b7df96c91f9c12dcc8a07bcb9ccacbed38856a # v7
         with:
           context: .github/docker
           file: .github/docker/Dockerfile.ci