feat: Add dashboard extension hook for third-party plugins (v0.4.6)

Add wp_prometheus_register_dashboards action hook allowing third-party
plugins to register their own Grafana dashboard templates.

- DashboardProvider: registration system with file/JSON support
- Security: path traversal protection, JSON validation
- Admin UI: "Extension" badge and plugin attribution
- Isolated mode support for dashboard registration hook

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -6,7 +6,13 @@ A WordPress plugin that provides a Prometheus-compatible `/metrics` endpoint wit
 
 - Prometheus-compatible authenticated `/metrics` endpoint
 - Default WordPress metrics (users, posts, comments, plugins)
+- Runtime metrics (HTTP requests, database queries)
+- Cron job and transient cache metrics
+- WooCommerce integration (products, orders, revenue)
+- Custom metric builder with admin UI
+- Grafana dashboard templates with download
 - Extensible by other plugins using hooks
+- Dashboard extension hook for third-party Grafana dashboards
 - Settings page under Settings > Metrics
 - Bearer token authentication
 - License management integration
@@ -154,6 +160,51 @@ $histogram = $collector->register_histogram( $name, $help, $labels, $buckets );
 $histogram->observe( $value, $labelValues );
 ```
 
+## Extending with Custom Dashboards (v0.4.6+)
+
+Add your own Grafana dashboard templates using the `wp_prometheus_register_dashboards` action:
+
+```php
+add_action( 'wp_prometheus_register_dashboards', function( $provider ) {
+    // File-based dashboard
+    $provider->register_dashboard( 'my-plugin-dashboard', array(
+        'title'       => __( 'My Plugin Metrics', 'my-plugin' ),
+        'description' => __( 'Dashboard for my custom metrics', 'my-plugin' ),
+        'icon'        => 'dashicons-chart-bar',
+        'file'        => MY_PLUGIN_PATH . 'assets/dashboards/my-dashboard.json',
+        'plugin'      => 'My Plugin Name',
+    ) );
+
+    // OR inline JSON dashboard
+    $provider->register_dashboard( 'dynamic-dashboard', array(
+        'title'       => __( 'Dynamic Dashboard', 'my-plugin' ),
+        'description' => __( 'Dynamically generated dashboard', 'my-plugin' ),
+        'icon'        => 'dashicons-admin-generic',
+        'json'        => json_encode( $dashboard_array ),
+        'plugin'      => 'My Plugin Name',
+    ) );
+} );
+```
+
+### Registration Parameters
+
+| Parameter | Required | Description |
+| --------- | -------- | ----------- |
+| `title` | Yes | Dashboard title displayed in admin |
+| `description` | No | Description shown below the title |
+| `icon` | No | Dashicon class (default: `dashicons-chart-line`) |
+| `file` | Yes* | Absolute path to JSON file |
+| `json` | Yes* | Inline JSON content |
+| `plugin` | No | Plugin name for attribution |
+
+*Either `file` or `json` is required, but not both.
+
+### Security Notes
+
+- File paths must be absolute and within `wp-content/`
+- Inline JSON is validated during registration
+- Third-party dashboards are marked with an "Extension" badge in the admin UI
+
 ## Development
 
 ### Build for Release