fix: exit codes match spec (0, 1, 2, 127) and silence usage on runtime errors

Exit 2 for missing required arguments, exit 127 for agent not found. SilenceUsage on all commands to avoid dumping usage on runtime errors.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/info.go

@@ -11,8 +11,9 @@ import (
 var infoCmd = &cobra.Command{
 	Use:   "info <query>",
 	Short: "Display metadata and path for a workspace",
-	Args:  cobra.ExactArgs(1),
-	RunE:  runInfo,
+	Args:         cobra.ExactArgs(1),
+	SilenceUsage: true,
+	RunE:         runInfo,
 }
 
 var infoAll bool