ci(release): attach Sigstore provenance bundle as a release asset (P21.1e) (#106)

OpenSSF Scorecard's Signed-Releases check scans the GitHub Release *assets* for
signed/provenance files (`*.intoto.jsonl`, `*.sig`, ...). It does not look at
GitHub's separate attestations store, so although the attestations job has signed
the published bytes since v0.4.0, the v0.4.0 release assets carried no provenance
file and the check stayed at 0.

Attach the Sigstore provenance bundle (already produced by
actions/attest-build-provenance) to the release as `wickra-<tag>.provenance.intoto.jsonl`:

- github-release now exposes its resolved tag as a job output.
- attestations `needs: github-release` (so the Release already exists), gains
  `contents: write`, gives the attest step an id, and uploads the bundle with
  `gh release upload --clobber` (idempotent on re-runs).

Publishes stay fully isolated — cargo/PyPI/npm all run upstream of github-release,
so a Sigstore hiccup here can never block or corrupt a publish; at worst the
release just lacks the provenance asset. Signed-Releases climbs over the next
releases as each tag carries the bundle.

.github/workflows/release.yml

@@ -556,6 +556,10 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write
+    # Expose the resolved tag so the attestations job can attach the provenance
+    # bundle to this same release without re-resolving it.
+    outputs:
+      tag: ${{ steps.tag.outputs.tag }}
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
@@ -640,18 +644,25 @@ jobs:
   # --------------------------------------------------------------------------
   attestations:
     name: Attest build provenance
-    needs: [cargo-publish, python-wheels, python-sdist]
+    needs: [cargo-publish, python-wheels, python-sdist, github-release]
     runs-on: ubuntu-latest
     # Signed SLSA build-provenance attestations for the published crates and
     # Python wheels/sdist. npm tarballs already carry inline Sigstore provenance
-    # from `npm publish --provenance`, so they are covered there. This job is
-    # isolated and runs *after* the publishes on the exact uploaded bytes, so a
-    # failure here can never block or corrupt a publish (same isolation that the
-    # SBOM step lacked before #79).
+    # from `npm publish --provenance`, so they are covered there.
+    #
+    # The job stays isolated from the *publishes*: cargo/PyPI/npm all run upstream
+    # of github-release, so a Sigstore hiccup here can never block or corrupt a
+    # publish (the isolation the SBOM step lacked before #79). It additionally
+    # `needs: github-release` so the GitHub Release already exists when it attaches
+    # the provenance bundle as a release asset (P21.1e) — OpenSSF Scorecard's
+    # Signed-Releases check scans release *assets* (*.intoto.jsonl), not GitHub's
+    # separate attestations store, so the bundle has to live on the release. A
+    # failure here still only costs the provenance asset; the release is already
+    # complete.
     permissions:
       id-token: write # OIDC for keyless Sigstore signing
       attestations: write # write the attestations to this repo
-      contents: read
+      contents: write # upload the provenance bundle as a release asset
     steps:
       - name: Download crate files
         uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
@@ -665,9 +676,35 @@ jobs:
           path: artifacts/python
           merge-multiple: true
       - name: Attest build provenance
+        id: attest
         uses: actions/attest-build-provenance@a2bbfa25375fe432b6a289bc6b6cd05ecd0c4c32 # v4.1.0
         with:
           subject-path: |
             artifacts/crates/*.crate
             artifacts/python/*.whl
-            artifacts/python/*.tar.gz
+            artifacts/python/*.tar.gz
+
+      # Attach the Sigstore provenance bundle to the GitHub Release as a
+      # `*.intoto.jsonl` asset so OpenSSF Scorecard's Signed-Releases check finds
+      # signed provenance on the release itself (P21.1e). attest-build-provenance
+      # writes a single JSONL bundle covering every subject above; copy it to a
+      # `.intoto.jsonl`-suffixed name and upload with --clobber so re-runs are
+      # idempotent. github.token has contents: write here, which is all gh needs.
+      - name: Attach provenance bundle to the GitHub Release
+        env:
+          GH_TOKEN: ${{ github.token }}
+          TAG: ${{ needs.github-release.outputs.tag }}
+          BUNDLE: ${{ steps.attest.outputs.bundle-path }}
+        run: |
+          if [ -z "$TAG" ]; then
+            echo "::error::no tag resolved from github-release; cannot attach provenance."
+            exit 1
+          fi
+          if [ -z "$BUNDLE" ] || [ ! -f "$BUNDLE" ]; then
+            echo "::error::attestation bundle not found at '$BUNDLE'."
+            exit 1
+          fi
+          dest="wickra-${TAG}.provenance.intoto.jsonl"
+          cp "$BUNDLE" "$dest"
+          echo "Uploading $dest to release $TAG"
+          gh release upload "$TAG" "$dest" --clobber --repo "${{ github.repository }}"