Revert to simple gt-mayor/gt-deacon session names

Reverts the session naming changes from PR #70. Multi-town support
on a single machine is not a real use case - rigs provide project
isolation, and true isolation should use containers/VMs.

Changes:
- MayorSessionName() and DeaconSessionName() no longer take townName parameter
- ParseSessionName() handles simple gt-mayor and gt-deacon formats
- Removed Town field from AgentIdentity and AgentSession structs
- Updated all callers and tests

Generated with Claude Code

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/daemon/lifecycle_test.go

@@ -184,9 +184,10 @@ func TestIdentityToSession_Mayor(t *testing.T) {
 	d, cleanup := testDaemonWithTown(t, "ai")
 	defer cleanup()
 
+	// Mayor session name is now fixed (one per machine, no town qualifier)
 	result := d.identityToSession("mayor")
-	if result != "gt-ai-mayor" {
-		t.Errorf("identityToSession('mayor') = %q, expected 'gt-ai-mayor'", result)
+	if result != "gt-mayor" {
+		t.Errorf("identityToSession('mayor') = %q, expected 'gt-mayor'", result)
 	}
 }