feat(ci): attest build provenance (#53)

Add actions/attest-build-provenance (pinned @e8998f9 # v2) to the
release job, attesting the same artifacts uploaded by
softprops/action-gh-release. Grants id-token: write and
attestations: write on that job.

Attested paths: artifacts/gsa-linux-x86_64.tar.gz

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

.github/workflows/release.yml

@@ -95,6 +95,8 @@ jobs:
     timeout-minutes: 30
     permissions:
       contents: write
+      id-token: write
+      attestations: write
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
@@ -115,6 +117,12 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Attest build provenance
+        uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be # v2
+        with:
+          subject-path: |
+            artifacts/gsa-linux-x86_64.tar.gz
+
   provenance:
     name: SLSA Provenance
     needs: [build]