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/polecat/manager.go

@@ -84,9 +84,9 @@ func (m *Manager) assigneeID(name string) string {
 }
 
 // agentBeadID returns the agent bead ID for a polecat.
-// Format: "gt-polecat-<rig>-<name>" (e.g., "gt-polecat-gastown-Toast")
+// Format: "gt-<rig>-polecat-<name>" (e.g., "gt-gastown-polecat-Toast")
 func (m *Manager) agentBeadID(name string) string {
-	return fmt.Sprintf("gt-polecat-%s-%s", m.rig.Name, name)
+	return beads.PolecatBeadID(m.rig.Name, name)
 }
 
 // getCleanupStatusFromBead reads the cleanup_status from the polecat's agent bead.