refactor(cmd): cobra UX improvements — NoArgs, MarkFlagFilename, preRun test fix

Implements three quick-win improvements identified in issue #4379 (cobra module
review):

1. Add Args: cobra.NoArgs to root command: positional arguments are not
   supported by awmg; without this, 'awmg config.toml' silently falls through
   to a confusing 'required flag(s) not set' error instead of a clear
   'unknown command' message.

2. Replace verbose RegisterFlagCompletionFunc callbacks with idiomatic cobra
   helpers: MarkFlagFilename("config", "toml"), MarkFlagDirname("log-dir"),
   MarkFlagDirname("payload-dir"), MarkFlagFilename("env", "env").
   Reduces ~20 lines of boilerplate to 4 lines with identical shell-completion
   behaviour.

3. Fix fragile preRun(nil, nil) in root_test.go: replace all 9 occurrences with
   preRun(&cobra.Command{}, nil). The current code is safe because preRun does
   not dereference cmd, but any future use of cmd.Context(), cmd.Flags(), etc.
   would cause a nil pointer panic. Using a real (empty) Command value prevents
   that footgun.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

internal/cmd/flags.go

@@ -57,25 +57,11 @@ func registerAllFlags(cmd *cobra.Command) {
 
 // registerFlagCompletions registers custom completion functions for flags
 func registerFlagCompletions(cmd *cobra.Command) {
-	// Custom completion for --config flag (complete with .toml files)
-	cmd.RegisterFlagCompletionFunc("config", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-		return []string{"toml"}, cobra.ShellCompDirectiveFilterFileExt
-	})
-
-	// Custom completion for --log-dir flag (complete with directories)
-	cmd.RegisterFlagCompletionFunc("log-dir", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-		return nil, cobra.ShellCompDirectiveFilterDirs
-	})
-
-	// Custom completion for --payload-dir flag (complete with directories)
-	cmd.RegisterFlagCompletionFunc("payload-dir", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-		return nil, cobra.ShellCompDirectiveFilterDirs
-	})
-
-	// Custom completion for --env flag (complete with .env files)
-	cmd.RegisterFlagCompletionFunc("env", func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
-		return []string{"env"}, cobra.ShellCompDirectiveFilterFileExt
-	})
+	// File and directory completions using idiomatic cobra helpers
+	cmd.MarkFlagFilename("config", "toml")
+	cmd.MarkFlagDirname("log-dir")
+	cmd.MarkFlagDirname("payload-dir")
+	cmd.MarkFlagFilename("env", "env")
 
 	// Enum completions for DIFC flags
 	cmd.RegisterFlagCompletionFunc("guards-mode", cobra.FixedCompletions(

internal/cmd/root.go

@@ -51,6 +51,7 @@ var rootCmd = &cobra.Command{
 	Version: cliVersion,
 	Long: `MCPG is a proxy server for Model Context Protocol (MCP) servers.
 It provides routing, aggregation, and management of multiple MCP backend servers.`,
+	Args:              cobra.NoArgs,
 	SilenceUsage:      true, // Don't show help on runtime errors
 	SilenceErrors:     true, // Prevent cobra from printing errors — Execute() caller handles display
 	PersistentPreRunE: preRun,

internal/cmd/root_test.go

@@ -40,23 +40,23 @@ func TestRunRequiresConfigSource(t *testing.T) {
 	t.Run("config file provided", func(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = false
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		// Should pass validation when --config is provided
 		assert.NoError(t, err, "Should not error when --config is provided")
 	})
 
 	t.Run("config stdin provided", func(t *testing.T) {
 		configFile = ""
 		configStdin = true
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		// Should pass validation when --config-stdin is provided
 		assert.NoError(t, err, "Should not error when --config-stdin is provided")
 	})
 
 	t.Run("both config file and stdin provided", func(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = true
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		// When both are provided, should pass validation
 		assert.NoError(t, err, "Should not error when both are provided")
 	})
@@ -78,15 +78,15 @@ func TestPreRunValidation(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = false
 		verbosity = 0
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		assert.NoError(t, err)
 	})
 
 	t.Run("validation passes with config stdin", func(t *testing.T) {
 		configFile = ""
 		configStdin = true
 		verbosity = 0
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		assert.NoError(t, err)
 	})
 
@@ -108,7 +108,7 @@ func TestPreRunValidation(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = false
 		verbosity = 1
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		assert.NoError(t, err)
 		// Level 1 doesn't set DEBUG env var
 		assert.Empty(t, os.Getenv(logger.EnvDebug))
@@ -129,7 +129,7 @@ func TestPreRunValidation(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = false
 		verbosity = 2
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		assert.NoError(t, err)
 		assert.Equal(t, "cmd:*,server:*,launcher:*", os.Getenv(logger.EnvDebug))
 	})
@@ -149,7 +149,7 @@ func TestPreRunValidation(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = false
 		verbosity = 3
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		assert.NoError(t, err)
 		assert.Equal(t, "*", os.Getenv(logger.EnvDebug))
 	})
@@ -169,7 +169,7 @@ func TestPreRunValidation(t *testing.T) {
 		configFile = "test.toml"
 		configStdin = false
 		verbosity = 2
-		err := preRun(nil, nil)
+		err := preRun(&cobra.Command{}, nil)
 		assert.NoError(t, err)
 		// Should not override existing DEBUG
 		assert.Equal(t, "custom:*", os.Getenv(logger.EnvDebug))