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/prime.go

@@ -315,8 +315,8 @@ func outputPrimeContext(ctx RoleContext) error {
 		TownName:      townName,
 		WorkDir:       ctx.WorkDir,
 		Polecat:       ctx.Polecat,
-		MayorSession:  session.MayorSessionName(townName),
-		DeaconSession: session.DeaconSessionName(townName),
+		MayorSession:  session.MayorSessionName(),
+		DeaconSession: session.DeaconSessionName(),
 	}
 
 	// Render and output