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/free-tests.yml

@@ -48,7 +48,7 @@ jobs:
     runs-on: ubicloud-standard-8
     timeout-minutes: 20
     steps:
-      - uses: actions/checkout@v7
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
         with:
           persist-credentials: false
 
@@ -144,7 +144,7 @@ jobs:
       # GREEN — that's the point — so its evidence must survive green runs.
       - name: Upload flake ledger
         if: always()
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: flake-ledger
           path: ${{ runner.temp }}/flake-ledger.jsonl
@@ -158,7 +158,7 @@ jobs:
       # need a local re-run, which fork contributors can't do on this image.
       - name: Upload shard logs on failure
         if: failure()
-        uses: actions/upload-artifact@v7
+        uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7
         with:
           name: free-test-shard-logs
           path: /tmp/gstack-free-test-*.log