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/imageref/imageref.go

@@ -2,7 +2,10 @@
 // and digest.
 package imageref
 
-import "strings"
+import (
+	"regexp"
+	"strings"
+)
 
 // Ref is an image reference split into its parts. Tag and Digest are empty
 // when the reference has none.
@@ -47,3 +50,8 @@ func (r Ref) String() string {
 	}
 	return s
 }
+
+var digestPattern = regexp.MustCompile(`^sha256:[0-9a-f]{64}$`)
+
+// IsDigest reports whether d is a well-formed sha256 digest.
+func IsDigest(d string) bool { return digestPattern.MatchString(d) }