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/polecat_cycle_test.go

@@ -64,14 +64,14 @@ func TestParsePolecatSessionName(t *testing.T) {
 		},
 		{
 			name:        "mayor session",
-			sessionName: "gt-mayor",
+			sessionName: "gt-ai-mayor",
 			wantRig:     "",
 			wantPolecat: "",
 			wantOk:      false,
 		},
 		{
 			name:        "deacon session",
-			sessionName: "gt-deacon",
+			sessionName: "gt-ai-deacon",
 			wantRig:     "",
 			wantPolecat: "",
 			wantOk:      false,