feat: Standardize agent bead naming to prefix-rig-role-name (gt-zvte2)

Implements canonical naming convention for agent bead IDs:
- Town-level: gt-mayor, gt-deacon (unchanged)
- Rig-level: gt-<rig>-witness, gt-<rig>-refinery (was gt-witness-<rig>)
- Named: gt-<rig>-crew-<name>, gt-<rig>-polecat-<name> (was gt-crew-<rig>-<name>)

Changes:
- Added AgentBeadID helper functions to internal/beads/beads.go
- Updated all ID generation call sites to use helpers
- Fixed session parsing in theme.go, statusline.go, agents.go
- Updated doctor check and fix to use canonical format
- Updated tests for new format

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

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

internal/cmd/agents.go

@@ -143,7 +143,7 @@ func categorizeSession(name string) *AgentSession {
 		return session
 	}
 
-	// Witness sessions use different format: gt-witness-<rig>
+	// Witness sessions: legacy format gt-witness-<rig> (fallback)
 	if strings.HasPrefix(suffix, "witness-") {
 		session.Type = AgentWitness
 		session.Rig = strings.TrimPrefix(suffix, "witness-")