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/dnd_test.go

@@ -5,12 +5,13 @@ import (
 )
 
 func TestAddressToAgentBeadID(t *testing.T) {
+	townName := "ai"
 	tests := []struct {
 		address  string
 		expected string
 	}{
-		{"mayor", "gt-mayor"},
-		{"deacon", "gt-deacon"},
+		{"mayor", "gt-ai-mayor"},
+		{"deacon", "gt-ai-deacon"},
 		{"gastown/witness", "gt-gastown-witness"},
 		{"gastown/refinery", "gt-gastown-refinery"},
 		{"gastown/alpha", "gt-gastown-polecat-alpha"},
@@ -24,9 +25,9 @@ func TestAddressToAgentBeadID(t *testing.T) {
 
 	for _, tt := range tests {
 		t.Run(tt.address, func(t *testing.T) {
-			got := addressToAgentBeadID(tt.address)
+			got := addressToAgentBeadID(tt.address, townName)
 			if got != tt.expected {
-				t.Errorf("addressToAgentBeadID(%q) = %q, want %q", tt.address, got, tt.expected)
+				t.Errorf("addressToAgentBeadID(%q, %q) = %q, want %q", tt.address, townName, got, tt.expected)
 			}
 		})
 	}