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/session/names.go

@@ -12,13 +12,17 @@ import (
 const Prefix = "gt-"
 
 // MayorSessionName returns the session name for the Mayor agent.
-func MayorSessionName() string {
-	return Prefix + "mayor"
+// The townName parameter allows multiple towns to run concurrently
+// without tmux session name collisions.
+func MayorSessionName(townName string) string {
+	return fmt.Sprintf("%s%s-mayor", Prefix, townName)
 }
 
 // DeaconSessionName returns the session name for the Deacon agent.
-func DeaconSessionName() string {
-	return Prefix + "deacon"
+// The townName parameter allows multiple towns to run concurrently
+// without tmux session name collisions.
+func DeaconSessionName(townName string) string {
+	return fmt.Sprintf("%s%s-deacon", Prefix, townName)
 }
 
 // WitnessSessionName returns the session name for a rig's Witness agent.