feat: gt doctor checks agent beads exist (gt-bq4wo)

Adds agent-beads-exist check to gt doctor:
- Verifies witness/refinery beads exist for each rig
- Verifies deacon/mayor beads exist in first rig
- Supports --fix to create missing agent beads
- Skips non-gt prefixes (beads library limitation)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/doctor.go

@@ -94,6 +94,7 @@ func runDoctor(cmd *cobra.Command, args []string) error {
 	d.Register(doctor.NewPatrolNotStuckCheck())
 	d.Register(doctor.NewPatrolPluginsAccessibleCheck())
 	d.Register(doctor.NewPatrolRolesHavePromptsCheck())
+	d.Register(doctor.NewAgentBeadsCheck())
 
 	// NOTE: StaleAttachmentsCheck removed - staleness detection belongs in Deacon molecule
 	// See gt-gaxo epic for ZFC cleanup rationale