refactor: move backend options into backend_config section

- Add BackendConfig map[string]any to Config struct
- Deprecate top-level output_path (backward compatible via EffectiveBackendConfig())
- NewHyprland now accepts opts map[string]any instead of raw outputPath
- Factory signature unified to func(*slog.Logger, map[string]any) for all backends
- Update README with backend_config docs and fix Sway skeleton signature
- Update example.yaml to show backend_config instead of output_path

internal/backend/hyprland.go

@@ -30,6 +30,7 @@ type hyprlandBackend struct {
 
 	// outputPath overrides the generated monitor config file path.
 	// Empty means the default: ~/.config/hypr/monitors.conf.
+	// Read from backend_config.output_path at construction time.
 	outputPath string
 
 	// conn is the current socket2 connection; guarded by mu.
@@ -40,9 +41,12 @@ type hyprlandBackend struct {
 // NewHyprland creates a Hyprland backend. The backend auto-detects the
 // Hyprland instance signature from the environment.
 //
-// outputPath overrides the generated monitor config file location.
-// When empty, defaults to ~/.config/hypr/monitors.conf.
-func NewHyprland(logger *slog.Logger, outputPath string) (Backend, error) {
+// opts carries backend-specific configuration from the config file's
+// backend_config section. Hyprland supports:
+//
+//	output_path — path to the generated monitor config file
+//	              (default: ~/.config/hypr/monitors.conf)
+func NewHyprland(logger *slog.Logger, opts map[string]any) (Backend, error) {
 	sig := os.Getenv("HYPRLAND_INSTANCE_SIGNATURE")
 	if sig == "" {
 		return nil, errors.New("HYPRLAND_INSTANCE_SIGNATURE is not set; is Hyprland running?")
@@ -57,6 +61,13 @@ func NewHyprland(logger *slog.Logger, outputPath string) (Backend, error) {
 		return nil, fmt.Errorf("socket2 not found at %s: %w", socketPath, err)
 	}
 
+	var outputPath string
+	if opts != nil {
+		if v, ok := opts["output_path"]; ok {
+			outputPath, _ = v.(string)
+		}
+	}
+
 	return &hyprlandBackend{logger: logger, outputPath: outputPath}, nil
 }