Add minimal GitHub build attestation workflow

.github/workflows/build-attest.yml

@@ -0,0 +1,53 @@
+name: Build + Attest
+
+on:
+  workflow_dispatch:
+  release:
+    types: [published]
+
+permissions:
+  contents: read
+  id-token: write
+  attestations: write
+
+jobs:
+  build-and-attest:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+          cache: pip
+
+      - name: Install build tooling
+        run: python -m pip install --upgrade pip build
+
+      - name: Build distributions
+        run: python -m build
+
+      - name: Upload built distributions
+        uses: actions/upload-artifact@v4
+        with:
+          name: python-dist
+          path: dist/*
+          if-no-files-found: error
+
+      - name: Generate build provenance attestation
+        uses: actions/attest@v4
+        with:
+          subject-path: "dist/*"
+
+      - name: Add verification hint
+        run: |
+          {
+            echo "## Verification"
+            echo
+            echo "Verify a downloaded artifact with:"
+            echo
+            echo '`gh attestation verify dist/<artifact-file> -R joy7758/agent-evidence`'
+          } >> "$GITHUB_STEP_SUMMARY"