feat: add json log format option

Add log_format config field ("text" default, "json" for structured
JSON output) to support machine-readable logging.

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

cmd/oubliette/main.go

@@ -34,7 +34,14 @@ func main() {
 		level.Set(slog.LevelInfo)
 	}
 
-	logger := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: level}))
+	var handler slog.Handler
+	opts := &slog.HandlerOptions{Level: level}
+	if cfg.LogFormat == "json" {
+		handler = slog.NewJSONHandler(os.Stderr, opts)
+	} else {
+		handler = slog.NewTextHandler(os.Stderr, opts)
+	}
+	logger := slog.New(handler)
 	slog.SetDefault(logger)
 
 	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)

internal/config/config.go

@@ -12,6 +12,7 @@ type Config struct {
 	SSH      SSHConfig  `toml:"ssh"`
 	Auth     AuthConfig `toml:"auth"`
 	LogLevel  string `toml:"log_level"`
+	LogFormat string `toml:"log_format"` // "text" (default) or "json"
 }
 
 type SSHConfig struct {
@@ -73,6 +74,9 @@ func applyDefaults(cfg *Config) {
 	if cfg.LogLevel == "" {
 		cfg.LogLevel = "info"
 	}
+	if cfg.LogFormat == "" {
+		cfg.LogFormat = "text"
+	}
 }
 
 func validate(cfg *Config) error {

oubliette.toml.example

@@ -1,4 +1,5 @@
 log_level = "info"
+log_format = "text" # "text" or "json"
 
 [ssh]
 listen_addr = ":2222"