Merge origin/main (phase 5, drift and merge) into feat/ledger-ui-services-agents-18

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

internal/master/store/migrations/0006_agent_cluster.sql

@@ -0,0 +1 @@
+ALTER TABLE agents ADD COLUMN swarm_cluster_id text NOT NULL DEFAULT '';