Conflicts:
- CLAUDE.md: keep both sets of gotchas (drift/merge from main, SSE ping and
swarm_cluster_id from this branch).
- internal/imageref/imageref_test.go: keep both TestIsDigest (main) and
TestIsRegistryHost (this branch).
- internal/master/web/server.go: take main's renderServiceError and make it
render through s.page, so service error pages get the sidebar nav data.
Also renumber this branch's migration 0005_agent_cluster.sql to
0006_agent_cluster.sql, after main's 0005_drift_merge.sql.
Claude-Session: https://claude.ai/code/session_01GAZsp3P7Qto6JnhipxWUJQ
