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_test.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadConfigExplicitPath(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "mcrctl.toml")
+	data := []byte(`server = "https://reg.example.com"
+grpc   = "reg.example.com:9443"
+token  = "file-token"
+ca_cert = "/path/to/ca.pem"
+`)
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := loadConfig(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if cfg.Server != "https://reg.example.com" {
+		t.Errorf("Server = %q, want %q", cfg.Server, "https://reg.example.com")
+	}
+	if cfg.GRPC != "reg.example.com:9443" {
+		t.Errorf("GRPC = %q, want %q", cfg.GRPC, "reg.example.com:9443")
+	}
+	if cfg.Token != "file-token" {
+		t.Errorf("Token = %q, want %q", cfg.Token, "file-token")
+	}
+	if cfg.CACert != "/path/to/ca.pem" {
+		t.Errorf("CACert = %q, want %q", cfg.CACert, "/path/to/ca.pem")
+	}
+}
+
+func TestLoadConfigMissingDefaultIsOK(t *testing.T) {
+	// Empty path triggers default search; missing file is not an error.
+	cfg, err := loadConfig("")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if cfg.Server != "" || cfg.GRPC != "" || cfg.Token != "" {
+		t.Errorf("expected zero config from missing default, got %+v", cfg)
+	}
+}
+
+func TestLoadConfigMissingExplicitIsError(t *testing.T) {
+	_, err := loadConfig("/nonexistent/mcrctl.toml")
+	if err == nil {
+		t.Fatal("expected error for missing explicit config path")
+	}
+}
+
+func TestLoadConfigInvalidTOML(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "bad.toml")
+	if err := os.WriteFile(path, []byte("not valid [[[ toml"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	_, err := loadConfig(path)
+	if err == nil {
+		t.Fatal("expected error for invalid TOML")
+	}
+}
+
+func TestLoadConfigPartial(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "mcrctl.toml")
+	data := []byte(`server = "https://reg.example.com"
+`)
+	if err := os.WriteFile(path, data, 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := loadConfig(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if cfg.Server != "https://reg.example.com" {
+		t.Errorf("Server = %q, want %q", cfg.Server, "https://reg.example.com")
+	}
+	if cfg.GRPC != "" {
+		t.Errorf("GRPC should be empty, got %q", cfg.GRPC)
+	}
+}