fix(ci): emit verifiable backend signature bundles (#11366)

Cosign v2.4.1 does not select the Sigstore bundle format by default, while LocalAI's verifier only consumes OCI bundle referrers. Request the format explicitly for both registries and guard the producer contract with a shell regression test.

Document strict backend integrity configuration and release-tag identities for operators.

Assisted-by: Codex:gpt-5

Co-authored-by: localai-org-maint-bot <306269227+localai-org-maint-bot@users.noreply.github.com>

.github/workflows/backend_merge.yml

@@ -71,8 +71,8 @@ jobs:
 
       # cosign signs each pushed manifest list with --recursive so the
       # index and every per-arch entry get an attached Sigstore bundle.
-      # Recent cosign releases always emit the new bundle format, so
-      # there's no extra CLI flag to opt into it.
+      # The pinned cosign v2 release needs --new-bundle-format explicitly;
+      # the verifier only consumes OCI 1.1 Sigstore bundle referrers.
       - name: Install cosign
         if: github.event_name != 'pull_request'
         uses: sigstore/cosign-installer@v3
@@ -159,6 +159,7 @@ jobs:
           # manifest before checking signatures need the per-arch
           # signatures, not just the list-level one.
           cosign sign --yes --recursive \
+            --new-bundle-format \
             --registry-referrers-mode=oci-1-1 \
             "quay.io/go-skynet/local-ai-backends@${digest}"
 
@@ -185,6 +186,7 @@ jobs:
           ' <<< "$DOCKER_METADATA_OUTPUT_JSON")
           digest=$(docker buildx imagetools inspect "$first_tag" --format '{{.Manifest.Digest}}')
           cosign sign --yes --recursive \
+            --new-bundle-format \
             --registry-referrers-mode=oci-1-1 \
             "localai/localai-backends@${digest}"