feat: add configuration management with YAML parsing and validation

Add internal/config package with Config struct (Server, Navidrome,
MusicBrainz, Telegram, Scanner sections), LoadConfig function for
YAML parsing, config validation (required fields, port range,
threshold range), and defaults. Include config.yaml.example matching
spec. Update main.go to use real config package instead of
placeholders. Add comprehensive tests covering valid configs,
defaults, missing files, malformed YAML, and validation boundaries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/naviwatcher/main.go

@@ -3,12 +3,13 @@ package main
 import (
 	"context"
 	"flag"
-	"fmt"
 	"log"
 	"os"
 	"os/signal"
 	"syscall"
 	"time"
+
+	"naviwatcher/internal/config"
 )
 
 const defaultConfigPath = "config.yaml"
@@ -19,7 +20,7 @@ func main() {
 
 	log.Println("NaviWatcher starting...")
 
-	cfg, err := loadConfig(*configPath)
+	cfg, err := config.LoadConfig(*configPath)
 	if err != nil {
 		log.Fatalf("Failed to load config: %v", err)
 	}
@@ -45,44 +46,7 @@ func main() {
 	log.Println("NaviWatcher stopped.")
 }
 
-// placeholderConfig mirrors the expected config structure for the skeleton phase.
-// Will be replaced by internal/config.Config in Task 2.
-type placeholderConfig struct {
-	Server struct {
-		Host string
-		Port int
-	}
-}
-
-func loadConfig(path string) (*placeholderConfig, error) {
-	data, err := os.ReadFile(path)
-	if err != nil {
-		return nil, fmt.Errorf("read config file: %w", err)
-	}
-
-	var cfg placeholderConfig
-	if err := parseYAML(data, &cfg); err != nil {
-		return nil, fmt.Errorf("parse config: %w", err)
-	}
-
-	if cfg.Server.Host == "" {
-		cfg.Server.Host = "0.0.0.0"
-	}
-	if cfg.Server.Port == 0 {
-		cfg.Server.Port = 8080
-	}
-
-	return &cfg, nil
-}
-
-// parseYAML is a temporary placeholder. Will be replaced by gopkg.in/yaml.v3 in Task 2.
-func parseYAML(data []byte, out *placeholderConfig) error {
-	// Minimal YAML parsing for skeleton: just handle "server:\n  host: ...\n  port: ..."
-	// This is intentionally simple and will be replaced by proper YAML parsing.
-	return nil
-}
-
-func run(ctx context.Context, cfg *placeholderConfig) error {
+func run(ctx context.Context, cfg *config.Config) error {
 	// Main application loop — blocks until context is cancelled.
 	// Business logic will be added in future tasks.
 	<-ctx.Done()