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/mayor.go

@@ -14,18 +14,9 @@ import (
 	"github.com/steveyegge/gastown/internal/workspace"
 )
 
-// getMayorSessionName returns the Mayor session name for the current workspace.
-// The session name includes the town name to avoid collisions between multiple HQs.
+// getMayorSessionName returns the Mayor session name.
 func getMayorSessionName() (string, error) {
-	townRoot, err := workspace.FindFromCwdOrError()
-	if err != nil {
-		return "", fmt.Errorf("not in a Gas Town workspace: %w", err)
-	}
-	townName, err := workspace.GetTownName(townRoot)
-	if err != nil {
-		return "", err
-	}
-	return session.MayorSessionName(townName), nil
+	return session.MayorSessionName(), nil
 }
 
 var mayorCmd = &cobra.Command{