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/constants/constants.go

@@ -90,13 +90,9 @@ const (
 )
 
 // Tmux session names.
+// Note: Mayor and Deacon session names are now dynamic (include town name).
+// Use session.MayorSessionName(townName) and session.DeaconSessionName(townName).
 const (
-	// SessionMayor is the tmux session name for the mayor.
-	SessionMayor = "gt-mayor"
-
-	// SessionDeacon is the tmux session name for the deacon.
-	SessionDeacon = "gt-deacon"
-
 	// SessionPrefix is the prefix for all Gas Town tmux sessions.
 	SessionPrefix = "gt-"
 )