feat(config): add yaml config with generic env override

Adds internal/config package using gopkg.in/yaml.v3 and reflection for generic environment variable overrides. Updates main.go to load config and set log level. Adds tests and example config.

internal/config/config_test.go

@@ -0,0 +1,43 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoad(t *testing.T) {
+	// Create a temporary config file
+	tmpDir := t.TempDir()
+	configFile := filepath.Join(tmpDir, "config.yaml")
+	
+	yamlContent := []byte(`
+log_level: "info"
+`)
+	if err := os.WriteFile(configFile, yamlContent, 0644); err != nil {
+		t.Fatalf("failed to write temp config file: %v", err)
+	}
+
+	t.Run("Load from YAML", func(t *testing.T) {
+		cfg, err := Load(configFile, "TEST")
+		if err != nil {
+			t.Fatalf("Load() error = %v", err)
+		}
+		if cfg.LogLevel != "info" {
+			t.Errorf("expected LogLevel 'info', got '%s'", cfg.LogLevel)
+		}
+	})
+
+	t.Run("Override from Env", func(t *testing.T) {
+		os.Setenv("TEST_LOG_LEVEL", "debug")
+		defer os.Unsetenv("TEST_LOG_LEVEL")
+
+		cfg, err := Load(configFile, "TEST")
+		if err != nil {
+			t.Fatalf("Load() error = %v", err)
+		}
+		if cfg.LogLevel != "debug" {
+			t.Errorf("expected LogLevel 'debug', got '%s'", cfg.LogLevel)
+		}
+	})
+}