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_test.go

@@ -49,15 +49,17 @@ func TestDeriveSessionName(t *testing.T) {
 			name: "mayor session",
 			envVars: map[string]string{
 				"GT_ROLE": "mayor",
+				"GT_TOWN": "ai",
 			},
-			expected: "gt-mayor",
+			expected: "gt-ai-mayor",
 		},
 		{
 			name: "deacon session",
 			envVars: map[string]string{
 				"GT_ROLE": "deacon",
+				"GT_TOWN": "ai",
 			},
-			expected: "gt-deacon",
+			expected: "gt-ai-deacon",
 		},
 		{
 			name:     "no env vars",
@@ -70,7 +72,7 @@ func TestDeriveSessionName(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			// Save and clear relevant env vars
 			saved := make(map[string]string)
-			envKeys := []string{"GT_ROLE", "GT_RIG", "GT_POLECAT", "GT_CREW"}
+			envKeys := []string{"GT_ROLE", "GT_RIG", "GT_POLECAT", "GT_CREW", "GT_TOWN"}
 			for _, key := range envKeys {
 				saved[key] = os.Getenv(key)
 				os.Unsetenv(key)