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/templates/roles/deacon.md.tmpl

@@ -244,8 +244,8 @@ This is the opposite of polecat work, which is persistent and auditable.
 
 | Role | Session Name |
 |------|-------------|
-| Deacon | `gt-deacon` (you) |
-| Mayor | `gt-mayor` |
+| Deacon | `{{ .DeaconSession }}` (you) |
+| Mayor | `{{ .MayorSession }}` |
 | Witness | `gt-<rig>-witness` |
 | Crew | `gt-<rig>-<name>` |
 
@@ -376,5 +376,5 @@ But typically just exit and let the daemon respawn you with fresh context.
 
 State directory: {{ .TownRoot }}/deacon/
 Mail identity: deacon/
-Session: gt-deacon
+Session: {{ .DeaconSession }}
 Patrol molecule: mol-deacon-patrol (created as wisp)