fix(stale): enable stale binary warning and fix source detection

- Add checkStaleBinaryWarning() call to persistentPreRun (was only in
  deprecated function)
- Fix GetRepoRoot() to look in correct location ($GT_ROOT/gastown/mayor/rig)
- Use hasGtSource() with os.Stat instead of shell test command

Agents will now see warnings when running gt with a stale binary.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/root.go

@@ -69,6 +69,11 @@ func persistentPreRun(cmd *cobra.Command, args []string) error {
 	// Get the root command name being run
 	cmdName := cmd.Name()
 
+	// Check for stale binary (warning only, doesn't block)
+	if !beadsExemptCommands[cmdName] {
+		checkStaleBinaryWarning()
+	}
+
 	// Check town root branch (warning only, non-blocking)
 	if !branchCheckExemptCommands[cmdName] {
 		warnIfTownRootOffMain()