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>

main.go

@@ -2,12 +2,23 @@ package main
 
 import (
 	"os"
+	"strings"
 
 	"github.com/warrenronsiek/ctask/cmd"
 )
 
 func main() {
 	if err := cmd.Execute(); err != nil {
+		msg := err.Error()
+		// Exit code 127 for agent command not found (per spec)
+		if strings.Contains(msg, "agent command not found") {
+			os.Exit(127)
+		}
+		// Exit code 2 for missing required arguments (per spec)
+		if strings.Contains(msg, "accepts") && strings.Contains(msg, "received 0") {
+			os.Exit(2)
+		}
 		os.Exit(1)
 	}
 }
+