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/doctor/orphan_check.go

@@ -10,7 +10,6 @@ import (
 
 	"github.com/steveyegge/gastown/internal/session"
 	"github.com/steveyegge/gastown/internal/tmux"
-	"github.com/steveyegge/gastown/internal/workspace"
 )
 
 // OrphanSessionCheck detects orphaned tmux sessions that don't match
@@ -57,12 +56,9 @@ func (c *OrphanSessionCheck) Run(ctx *CheckContext) *CheckResult {
 	// Get list of valid rigs
 	validRigs := c.getValidRigs(ctx.TownRoot)
 
-	// Get dynamic session names for mayor/deacon
-	var mayorSession, deaconSession string
-	if townName, err := workspace.GetTownName(ctx.TownRoot); err == nil {
-		mayorSession = session.MayorSessionName(townName)
-		deaconSession = session.DeaconSessionName(townName)
-	}
+	// Get session names for mayor/deacon
+	mayorSession := session.MayorSessionName()
+	deaconSession := session.DeaconSessionName()
 
 	// Check each session
 	var orphans []string