feat(ci): attest build provenance (#287)

## Estate attestation rollout

Adds GitHub native build-provenance attestation to
`.github/workflows/ghcr-publish.yml`.

**Push mechanism:** nerdctl (`nerdctl build` + `nerdctl push` to
ghcr.io). This workflow pushes **two distinct artifacts**: the minimal
image (`:${{ github.sha }}` / `:latest`, from
`.containerization/Containerfile`) and the `:full` image (from
`.containerization/Containerfile.full`).

**How the digest is obtained:** `nerdctl push` has no `--digestfile`
flag, so after the pushes the workflow reads each image's real
`sha256:...` digest back from the registry with `skopeo inspect --format
'{{.Digest}}'` (pre-installed on `ubuntu-latest`), authenticating with
`GITHUB_TOKEN`. Because the minimal and full images are different
artifacts with different digests, each gets its own attestation step.

**Changes:**
- The job already declared `id-token: write` + `attestations: write` —
no permission change needed.
- Captured the minimal and full image digests via `skopeo inspect`.
- Added two `actions/attest-build-provenance@e8998f94…` (v2, pinned)
steps (minimal + full), both with `push-to-registry: true`.

Verify with:
```
gh attest verify oci://ghcr.io/hyperpolymath/echidna:<tag> --repo hyperpolymath/echidna
```

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: hyperpolymath

.github/workflows/ghcr-publish.yml

@@ -65,10 +65,41 @@ jobs:
           sudo nerdctl build -f .containerization/Containerfile.full -t ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:full .
 
       - name: Push images
+        id: push
         run: |
           sudo nerdctl push ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ github.sha }}
           sudo nerdctl push ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:latest
           sudo nerdctl push ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:full
+          # nerdctl push has no --digestfile, so read the real sha256 of each
+          # pushed image back from the registry with skopeo (pre-installed on
+          # ubuntu-latest). The minimal (:sha/:latest) and :full images are
+          # distinct artifacts, so each gets its own attestation.
+          MIN_DIGEST="$(skopeo inspect --format '{{.Digest}}' \
+            --creds "${{ github.actor }}:${{ secrets.GITHUB_TOKEN }}" \
+            docker://${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ github.sha }})"
+          FULL_DIGEST="$(skopeo inspect --format '{{.Digest}}' \
+            --creds "${{ github.actor }}:${{ secrets.GITHUB_TOKEN }}" \
+            docker://${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:full)"
+          echo "min_digest=$MIN_DIGEST" >> "$GITHUB_OUTPUT"
+          echo "full_digest=$FULL_DIGEST" >> "$GITHUB_OUTPUT"
+
+      # GitHub native artifact attestation (build provenance) for the pushed
+      # images. Verify with:
+      #   gh attest verify oci://ghcr.io/${{ github.repository }}:<tag> \
+      #     --repo ${{ github.repository }}
+      - name: Attest container provenance (minimal image)
+        uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be # v2
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.push.outputs.min_digest }}
+          push-to-registry: true
+
+      - name: Attest container provenance (full image)
+        uses: actions/attest-build-provenance@e8998f949152b193b063cb0ec769d69d929409be # v2
+        with:
+          subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+          subject-digest: ${{ steps.push.outputs.full_digest }}
+          push-to-registry: true
 
       - name: Tag release version
         if: github.event_name == 'release'