fix(ci): SHA-pin dependency-review; the secret gate fails closed without a report

dependency-review.yml rode mutable refs (@v4 resolves to a BRANCH on
that repo) inside the one workflow whose job is supply-chain hygiene —
now commit-pinned like its siblings, with dependabot keeping the pins
fresh. gate-secret-scan.mjs crashed with an unhandled EPIPE on
oversize diffs (the designed report.oversize branch was unreachable:
the scanner emits no JSON on refusal) — the pipe write now tolerates
early exit and a missing report is an explicit fail-closed exit 1.
Oversize + broken-scanner legs pinned.

.github/workflows/dependency-review.yml

@@ -24,8 +24,8 @@ jobs:
       contents: read
       pull-requests: write
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/dependency-review-action@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/dependency-review-action@2031cfc080254a8a887f58cffee85186f0e49e48 # v4.9.0
         with:
           fail-on-severity: high
           fail-on-scopes: runtime, development