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/session/names_test.go

@@ -8,42 +8,20 @@ import (
 )
 
 func TestMayorSessionName(t *testing.T) {
-	tests := []struct {
-		townName string
-		want     string
-	}{
-		{"ai", "gt-ai-mayor"},
-		{"alpha", "gt-alpha-mayor"},
-		{"gastown", "gt-gastown-mayor"},
-		{"", "gt--mayor"}, // empty town name
-	}
-	for _, tt := range tests {
-		t.Run(tt.townName, func(t *testing.T) {
-			got := MayorSessionName(tt.townName)
-			if got != tt.want {
-				t.Errorf("MayorSessionName(%q) = %q, want %q", tt.townName, got, tt.want)
-			}
-		})
+	// Mayor session name is now fixed (one per machine)
+	want := "gt-mayor"
+	got := MayorSessionName()
+	if got != want {
+		t.Errorf("MayorSessionName() = %q, want %q", got, want)
 	}
 }
 
 func TestDeaconSessionName(t *testing.T) {
-	tests := []struct {
-		townName string
-		want     string
-	}{
-		{"ai", "gt-ai-deacon"},
-		{"alpha", "gt-alpha-deacon"},
-		{"gastown", "gt-gastown-deacon"},
-		{"", "gt--deacon"}, // empty town name
-	}
-	for _, tt := range tests {
-		t.Run(tt.townName, func(t *testing.T) {
-			got := DeaconSessionName(tt.townName)
-			if got != tt.want {
-				t.Errorf("DeaconSessionName(%q) = %q, want %q", tt.townName, got, tt.want)
-			}
-		})
+	// Deacon session name is now fixed (one per machine)
+	want := "gt-deacon"
+	got := DeaconSessionName()
+	if got != want {
+		t.Errorf("DeaconSessionName() = %q, want %q", got, want)
 	}
 }