ci(release): publish Sigstore attestation bundles

Ship per-target and SHA256SUMS .intoto.jsonl files with the release
so provenance is downloadable for manual and offline checks (#1267).

.github/workflows/build-release.yml

@@ -98,8 +98,11 @@ jobs:
 
       # Sigstore keyless provenance (GitHub OIDC). Attest the published archive
       # and wheel on the runner that built them so the installer can pin
-      # usestrix/strix/.github/workflows/build-release.yml.
+      # usestrix/strix/.github/workflows/build-release.yml. The bundle is also
+      # copied into dist/release/ so it ships as a release asset for manual /
+      # offline verification (#1267).
       - name: Attest Unix archive and wheel
+        id: attest-unix
         if: runner.os != 'Windows'
         uses: actions/attest@1e69f48acb82d1966a394da916b4c1698aa569d6  # v4.2.2
         with:
@@ -108,6 +111,7 @@ jobs:
             dist/*.whl
 
       - name: Attest Windows archive and wheel
+        id: attest-windows
         if: runner.os == 'Windows'
         uses: actions/attest@1e69f48acb82d1966a394da916b4c1698aa569d6  # v4.2.2
         with:
@@ -115,12 +119,23 @@ jobs:
             dist/release/*.zip
             dist/*.whl
 
+      - name: Publish attestation bundle
+        shell: bash
+        env:
+          # Only one of the attest steps runs; the other output is empty.
+          BUNDLE_PATH: ${{ steps.attest-unix.outputs.bundle-path }}${{ steps.attest-windows.outputs.bundle-path }}
+        run: |
+          test -n "$BUNDLE_PATH"
+          test -f "$BUNDLE_PATH"
+          cp "$BUNDLE_PATH" "dist/release/strix-${{ matrix.target }}.intoto.jsonl"
+
       - uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02  # v4.6.2
         with:
           name: strix-${{ matrix.target }}
           path: |
             dist/release/*.tar.gz
             dist/release/*.zip
+            dist/release/*.intoto.jsonl
             dist/*.whl
           if-no-files-found: error
 
@@ -138,11 +153,9 @@ jobs:
           path: release
           merge-multiple: true
 
-      # Same-origin integrity manifest for every release asset. Hashes use bare
-      # basenames (no directory prefix) so the installer can match the file it
-      # downloaded into a temp dir. Detects corruption / single-asset swap.
-      # Sigstore provenance on the archive itself is what binds the bytes to
-      # this workflow (#1267).
+      # Same-origin integrity manifest for product assets (archives + wheels).
+      # Provenance bundles are published alongside the release but excluded here
+      # so SHA256SUMS stays a stable product-byte list the installer greps.
       - name: Generate SHA256SUMS
         working-directory: release
         shell: bash
@@ -150,19 +163,29 @@ jobs:
           : > SHA256SUMS
           while IFS= read -r -d '' f; do
             ( cd "$(dirname "$f")" && sha256sum "$(basename "$f")" )
-          done < <(find . -type f ! -name SHA256SUMS -print0 | sort -z) >> SHA256SUMS
+          done < <(find . -type f ! -name SHA256SUMS ! -name '*.intoto.jsonl' -print0 | sort -z) >> SHA256SUMS
           echo "=== SHA256SUMS ==="
           cat SHA256SUMS
 
       - name: Attest SHA256SUMS
+        id: attest-sums
         uses: actions/attest@1e69f48acb82d1966a394da916b4c1698aa569d6  # v4.2.2
         with:
           subject-path: release/SHA256SUMS
 
+      - name: Publish SHA256SUMS attestation bundle
+        shell: bash
+        env:
+          BUNDLE_PATH: ${{ steps.attest-sums.outputs.bundle-path }}
+        run: |
+          test -n "$BUNDLE_PATH"
+          test -f "$BUNDLE_PATH"
+          cp "$BUNDLE_PATH" release/SHA256SUMS.intoto.jsonl
+
       - name: Create Release
         uses: softprops/action-gh-release@3bb12739c298aeb8a4eeaf626c5b8d85266b0e65  # v2.6.2
         with:
           prerelease: ${{ !startsWith(github.ref, 'refs/tags/') }}
           generate_release_notes: true
-          # release/** already includes release/SHA256SUMS generated above.
+          # Includes SHA256SUMS and *.intoto.jsonl provenance bundles.
           files: release/**