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/new.go

@@ -13,8 +13,9 @@ var newCmd = &cobra.Command{
 	Use:   "new [title]",
 	Short: "Create a new task workspace and launch the agent",
 	Long:  "Create a new task workspace and launch the agent. If title is omitted, generates task-HHMMSS.",
-	Args:  cobra.MaximumNArgs(1),
-	RunE:  runNew,
+	Args:         cobra.MaximumNArgs(1),
+	SilenceUsage: true,
+	RunE:         runNew,
 }
 
 var (