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.

cmd/app/main.go

@@ -2,15 +2,30 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"log/slog"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
+
+	"github.com/placeholder/golang-template/internal/config"
 )
 
 func main() {
-	logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
+	// Load Configuration
+	cfg, err := config.Load("config/config.yaml", "APP")
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "failed to load config: %v\n", err)
+		os.Exit(1)
+	}
+
+	// Setup Logger
+	opts := &slog.HandlerOptions{
+		Level: parseLogLevel(cfg.LogLevel),
+	}
+	logger := slog.New(slog.NewJSONHandler(os.Stdout, opts))
 	slog.SetDefault(logger)
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -31,6 +46,21 @@ func main() {
 	}
 }
 
+func parseLogLevel(level string) slog.Level {
+	switch strings.ToLower(level) {
+	case "debug":
+		return slog.LevelDebug
+	case "info":
+		return slog.LevelInfo
+	case "warn":
+		return slog.LevelWarn
+	case "error":
+		return slog.LevelError
+	default:
+		return slog.LevelInfo
+	}
+}
+
 func run(ctx context.Context) error {
 	slog.Info("Starting application...")