feat(ci): attest build provenance

Add GitHub-native build-provenance attestation (actions/attest-build-provenance@v2,
SHA-pinned) to the release workflow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/release.yml

@@ -88,6 +88,10 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 15
     needs: [build, sbom]
+    permissions:
+      contents: write
+      id-token: write
+      attestations: write
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v4
       - name: Download artifacts
@@ -100,6 +104,10 @@ jobs:
           find release-artifacts -type f -exec cp {} release/ \;
           cd release
           sha256sum * > checksums.txt
+      - name: Attest build provenance
+        uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be # v2
+        with:
+          subject-path: 'release/*'
       - name: Extract version
         id: version
         run: |