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/statusline_test.go

@@ -16,12 +16,12 @@ func TestCategorizeSessionRig(t *testing.T) {
 		{"gt-gastown-crew-max", "gastown"},
 		{"gt-myrig-crew-user", "myrig"},
 
-		// Witness sessions (actual format: gt-witness-<rig>)
-		{"gt-witness-gastown", "gastown"},
-		{"gt-witness-myrig", "myrig"},
-		// Legacy format still works as fallback
+		// Witness sessions (canonical format: gt-<rig>-witness)
 		{"gt-gastown-witness", "gastown"},
 		{"gt-myrig-witness", "myrig"},
+		// Legacy format still works as fallback
+		{"gt-witness-gastown", "gastown"},
+		{"gt-witness-myrig", "myrig"},
 
 		// Refinery sessions
 		{"gt-gastown-refinery", "gastown"},
@@ -61,8 +61,8 @@ func TestCategorizeSessionType(t *testing.T) {
 		{"gt-a-b", AgentPolecat},
 
 		// Non-polecat sessions
-		{"gt-witness-gastown", AgentWitness}, // actual format
-		{"gt-gastown-witness", AgentWitness}, // legacy fallback
+		{"gt-gastown-witness", AgentWitness}, // canonical format
+		{"gt-witness-gastown", AgentWitness}, // legacy fallback
 		{"gt-gastown-refinery", AgentRefinery},
 		{"gt-gastown-crew-max", AgentCrew},
 		{"gt-myrig-crew-user", AgentCrew},