fix: Make Mayor/Deacon session names include town name

Session names `gt-mayor` and `gt-deacon` were hardcoded, causing tmux
session name collisions when running multiple towns simultaneously.

Changed to `gt-{town}-mayor` and `gt-{town}-deacon` format (e.g.,
`gt-ai-mayor`) to allow concurrent multi-town operation.

Key changes:
- session.MayorSessionName() and DeaconSessionName() now take townName param
- Added workspace.GetTownName() helper to load town name from config
- Updated all callers in cmd/, daemon/, doctor/, mail/, rig/, templates/
- Updated tests with new session name format
- Bead IDs remain unchanged (already scoped by .beads/ directory)

Fixes #60

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

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

internal/cmd/costs.go

@@ -618,12 +618,13 @@ func runCostsRecord(cmd *cobra.Command, args []string) error {
 //   - Polecats: gt-{rig}-{polecat} (e.g., gt-gastown-toast)
 //   - Crew: gt-{rig}-crew-{crew} (e.g., gt-gastown-crew-max)
 //   - Witness/Refinery: gt-{rig}-{role} (e.g., gt-gastown-witness)
-//   - Mayor/Deacon: gt-{role} (e.g., gt-mayor)
+//   - Mayor/Deacon: gt-{town}-{role} (e.g., gt-ai-mayor)
 func deriveSessionName() string {
 	role := os.Getenv("GT_ROLE")
 	rig := os.Getenv("GT_RIG")
 	polecat := os.Getenv("GT_POLECAT")
 	crew := os.Getenv("GT_CREW")
+	town := os.Getenv("GT_TOWN")
 
 	// Polecat: gt-{rig}-{polecat}
 	if polecat != "" && rig != "" {
@@ -635,9 +636,9 @@ func deriveSessionName() string {
 		return fmt.Sprintf("gt-%s-crew-%s", rig, crew)
 	}
 
-	// Global roles without rig: gt-{role}
-	if role != "" && rig == "" {
-		return fmt.Sprintf("gt-%s", role)
+	// Town-level roles (mayor, deacon): gt-{town}-{role}
+	if (role == "mayor" || role == "deacon") && town != "" {
+		return fmt.Sprintf("gt-%s-%s", town, role)
 	}
 
 	// Rig-based roles (witness, refinery): gt-{rig}-{role}