security(release): sign GitHub Release artifacts with Sigstore (keyless)

publish.yml github-release job now signs the wheel + sdist via
sigstore/gh-action-sigstore-python (pinned v3.3.0 / 04cffa1d), keyless OIDC,
and attaches the .sigstore.json bundles to the GitHub Release through the
existing dist/* glob. Added id-token: write to the job for OIDC.

PyPI uploads were already attested (Trusted Publishing); Scorecard's
Signed-Releases check inspects GitHub Releases, which only carried bare wheels
-> score 0. .sigstore.json is in Scorecard's recognized signatureExtensions.
Verified: action globs ./dist/*.whl ./dist/*.tar.gz (action.py:202), auto-attach
gated on release-event (we trigger on push:tags) so we upload via dist/* and set
release-signing-artifacts:false. First live proof = next v* tag.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/publish.yml

@@ -41,12 +41,24 @@ jobs:
     runs-on: ubuntu-latest
     permissions:
       contents: write
+      id-token: write # keyless Sigstore signing (OIDC); no signing key exists
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
         with:
           name: dist
           path: dist/
+      - name: Sign artifacts with Sigstore (keyless, OIDC)
+        # Produces dist/<artifact>.sigstore.json bundles next to each wheel/sdist.
+        # The 'gh release create dist/*' step below then attaches them to the
+        # GitHub Release, which is where Scorecard's Signed-Releases check looks.
+        # release-signing-artifacts is disabled: the action's own auto-attach only
+        # fires on a 'release: published' event, but we trigger on 'push: tags',
+        # so we upload the bundles ourselves via the dist/* glob.
+        uses: sigstore/gh-action-sigstore-python@04cffa1d795717b140764e8b640de88853c92acc # v3.3.0
+        with:
+          inputs: ./dist/*.tar.gz ./dist/*.whl
+          release-signing-artifacts: false
       - name: Extract latest CHANGELOG section
         run: |
           # Grab the topmost "## [...]" block from CHANGELOG.md as release notes.