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/cmd/statusline_test.go

@@ -31,8 +31,8 @@ func TestCategorizeSessionRig(t *testing.T) {
 		{"gt-a-b", "a"}, // minimum valid
 
 		// Town-level agents (no rig)
-		{"gt-ai-mayor", ""},
-		{"gt-ai-deacon", ""},
+		{"gt-mayor", ""},
+		{"gt-deacon", ""},
 	}
 
 	for _, tt := range tests {
@@ -68,8 +68,8 @@ func TestCategorizeSessionType(t *testing.T) {
 		{"gt-myrig-crew-user", AgentCrew},
 
 		// Town-level agents
-		{"gt-ai-mayor", AgentMayor},
-		{"gt-ai-deacon", AgentDeacon},
+		{"gt-mayor", AgentMayor},
+		{"gt-deacon", AgentDeacon},
 	}
 
 	for _, tt := range tests {