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

README.md

@@ -166,6 +166,25 @@ Two match modes in the `external` list:
 
 Run `hyprctl monitors all` to see your monitor names and descriptions.
 
+### Backend-specific configuration
+
+Use the `backend_config` section for compositor-specific options:
+
+```yaml
+backend_config:
+  output_path: ~/.config/hypr/custom-monitors.conf   # override generated config path
+```
+
+**Hyprland keys:**
+
+| Key | Type | Default | Description |
+|-----|------|---------|-------------|
+| `output_path` | string | `~/.config/hypr/monitors.conf` | Path to the generated monitor config file. Supports `~` expansion. |
+
+The deprecated top-level `output_path` key still works — `backend_config` takes priority if both are set.
+
+When adding new compositor backends, their options go into the same `backend_config` section without changing the core config structure.
+
 ### Hook commands
 
 Hooks are shell commands executed via `sh -c`. Each command gets a **30-second timeout**. Commands run in parallel — one slow hook won't block others.
@@ -273,8 +292,9 @@ package backend
 
 type swayBackend struct { logger *slog.Logger }
 
-func NewSway(logger *slog.Logger) (Backend, error) {
+func NewSway(logger *slog.Logger, opts map[string]any) (Backend, error) {
     // Check if SWAYSOCK is set and swaymsg is available
+    // opts carries backend_config from the config file
     return &swayBackend{logger: logger}, nil
 }