feat(logger): add structured logging with rotation

- add `logger.go` with json structured logging using `slog`
- implement automatic log file rotation (max 3 files)
- support custom log paths and xdg config directory
- add `Load()`, `Debug()`, `Info()`, `Warn()`, `Error()` functions
- add tests covering all logging functions and rotation logic
- add default log path generation with timestamp naming

Author: tharropoulos

internal/logger/logger.go

@@ -0,0 +1,147 @@
+package logger
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+	"path/filepath"
+	"sort"
+	"time"
+)
+
+var Logger *slog.Logger
+
+const maxLogFiles = 3
+
+func Load(logPath string) error {
+	var logFilePath string
+	var err error
+
+	if logPath != "" {
+		logFilePath = logPath
+	} else {
+		logFilePath, err = getDefaultLogPath()
+		if err != nil {
+			return fmt.Errorf("failed to get default log path: %w", err)
+		}
+	}
+
+	return initLogger(logFilePath)
+}
+
+func initLogger(logPath string) error {
+	logDir := filepath.Dir(logPath)
+	if err := os.MkdirAll(logDir, 0755); err != nil {
+		return fmt.Errorf("failed to create log directory: %w", err)
+	}
+
+	defaultLogPath, err := getDefaultLogPath()
+	if err == nil && filepath.Dir(defaultLogPath) == logDir {
+		if err := rotateLogFiles(logDir); err != nil {
+			return fmt.Errorf("failed to rotate log files: %w", err)
+		}
+	}
+
+	logFile, err := os.OpenFile(logPath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0644)
+	if err != nil {
+		return fmt.Errorf("failed to open log file: %w", err)
+	}
+
+	handler := slog.NewJSONHandler(logFile, &slog.HandlerOptions{
+		Level:     slog.LevelDebug,
+		AddSource: true,
+	})
+
+	Logger = slog.New(handler)
+
+	Info("Logger initialized", "log_file", logPath, "format", "json")
+	return nil
+}
+
+func rotateLogFiles(logDir string) error {
+	pattern := filepath.Join(logDir, "*.kyma.log")
+	matches, err := filepath.Glob(pattern)
+	if err != nil {
+		return fmt.Errorf("failed to find log files: %w", err)
+	}
+
+	if len(matches) < maxLogFiles {
+		return nil
+	}
+
+	type logFile struct {
+		path    string
+		modTime time.Time
+	}
+
+	var logFiles []logFile
+	for _, match := range matches {
+		info, err := os.Stat(match)
+		if err != nil {
+			continue
+		}
+		logFiles = append(logFiles, logFile{
+			path:    match,
+			modTime: info.ModTime(),
+		})
+	}
+
+	sort.Slice(logFiles, func(i, j int) bool {
+		return logFiles[i].modTime.Before(logFiles[j].modTime)
+	})
+
+	filesToRemove := len(logFiles) - (maxLogFiles - 1)
+	for i := range filesToRemove {
+		if err := os.Remove(logFiles[i].path); err != nil {
+			fmt.Fprintf(os.Stderr, "Warning: failed to remove old log file %s: %v\n", logFiles[i].path, err)
+		}
+	}
+
+	return nil
+}
+
+func getDefaultLogPath() (string, error) {
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", fmt.Errorf("failed to get user home directory: %w", err)
+	}
+
+	// Check XDG_CONFIG_HOME first, then fall back to ~/.config
+	xdgConfigHome := os.Getenv("XDG_CONFIG_HOME")
+	var configDir string
+	if xdgConfigHome != "" {
+		configDir = filepath.Join(xdgConfigHome, "kyma")
+	} else {
+		configDir = filepath.Join(home, ".config", "kyma")
+	}
+
+	logsDir := filepath.Join(configDir, "logs")
+	timestamp := time.Now().Format("2006-01-02_15-04-05")
+	logFile := fmt.Sprintf("%s.kyma.log", timestamp)
+
+	return filepath.Join(logsDir, logFile), nil
+}
+
+func Debug(msg string, keyvals ...any) {
+	if Logger != nil {
+		Logger.Debug(msg, keyvals...)
+	}
+}
+
+func Info(msg string, keyvals ...any) {
+	if Logger != nil {
+		Logger.Info(msg, keyvals...)
+	}
+}
+
+func Warn(msg string, keyvals ...any) {
+	if Logger != nil {
+		Logger.Warn(msg, keyvals...)
+	}
+}
+
+func Error(msg string, keyvals ...any) {
+	if Logger != nil {
+		Logger.Error(msg, keyvals...)
+	}
+}