Add TOML config file support to mcrctl

Loads defaults from ~/.config/mcrctl.toml (or XDG_CONFIG_HOME).
Resolution order: flag > env (MCR_TOKEN) > config file.
Adds --config flag to specify an explicit path.

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

cmd/mcrctl/config.go

@@ -0,0 +1,56 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	toml "github.com/pelletier/go-toml/v2"
+)
+
+// config holds CLI defaults loaded from a TOML file.
+type config struct {
+	Server string `toml:"server"`
+	GRPC   string `toml:"grpc"`
+	Token  string `toml:"token"`
+	CACert string `toml:"ca_cert"`
+}
+
+// loadConfig reads a TOML config file from the given path. If path is
+// empty it searches the default location (~/.config/mcrctl.toml). A
+// missing file at the default location is not an error — an explicit
+// --config path that doesn't exist is.
+func loadConfig(path string) (config, error) {
+	explicit := path != ""
+	if !explicit {
+		path = defaultConfigPath()
+	}
+
+	data, err := os.ReadFile(path) //nolint:gosec // operator-supplied config path
+	if err != nil {
+		if !explicit && os.IsNotExist(err) {
+			return config{}, nil
+		}
+		return config{}, fmt.Errorf("reading config: %w", err)
+	}
+
+	var cfg config
+	if err := toml.Unmarshal(data, &cfg); err != nil {
+		return config{}, fmt.Errorf("parsing config %s: %w", path, err)
+	}
+	return cfg, nil
+}
+
+// defaultConfigPath returns ~/.config/mcrctl.toml, respecting
+// XDG_CONFIG_HOME if set.
+func defaultConfigPath() string {
+	dir := os.Getenv("XDG_CONFIG_HOME")
+	if dir == "" {
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "mcrctl.toml"
+		}
+		dir = filepath.Join(home, ".config")
+	}
+	return filepath.Join(dir, "mcrctl.toml")
+}