cluster: mDNS discovery with a shared token, prefix-cache routing, Prometheus /metrics + per-key usage, node logs and re-push on return, GGUF row split, cosign-signed images

- codai/cluster/discovery.py: _coderai._tcp announce + browse (zeroconf), HMAC
  fingerprint of cluster.token as membership; members auto-join as nodes,
  listed nodes with a blank key get the shared token; nodes and engines accept
  the cluster token (front /cluster/*, engine BearerAuth); env overrides for
  capability images (CODERAI_CLUSTER_TOKEN/DISCOVERY/NODE_NAME/ADVERTISE_HOST);
  coderai-docker --host-network
- frontproxy/affinity.py + router prefer=: a conversation goes back to the
  engine/node that served it when the model is resident in several places;
  engine_for_model now picks the least busy holder
- frontproxy/metrics.py: /metrics (requests per key/model/kind/status/engine,
  engines/nodes up, VRAM, in-flight, RPC, discovery, RunPod spend), usage
  tables on the Cluster page, server.metrics_public
- node logs: /cluster/logs on the node, /admin/api/cluster/logs on the head,
  log viewer on the Cluster page; a node that comes back gets its assigned
  models pushed again
- per-model split_mode=row (llama.cpp tensor parallel over local + RPC devices);
  vLLM TP may span nodes (documented)
- packaging/sign-images.sh + packaging/cosign.pub: all 21 GHCR images signed
- docs/cluster.md §§ discovery, 7 prefix routing, 9 operating, 10 accelerators
  evaluation; tests/test_cluster_ops.py (11)
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKm7Z56iKjHgNkHc6VhWKz