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/resume.go

@@ -12,10 +12,11 @@ import (
 )
 
 var resumeCmd = &cobra.Command{
-	Use:   "resume <query>",
-	Short: "Reopen an existing workspace and launch the agent",
-	Args:  cobra.ExactArgs(1),
-	RunE:  runResume,
+	Use:          "resume <query>",
+	Short:        "Reopen an existing workspace and launch the agent",
+	Args:         cobra.ExactArgs(1),
+	SilenceUsage: true,
+	RunE:         runResume,
 }
 
 var (