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>

CHANGELOG.md

@@ -5,6 +5,23 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.4.6] - 2026-02-03
+
+### Added
+
+- Dashboard extension hook `wp_prometheus_register_dashboards` for third-party plugins
+- Third-party plugins can now register their own Grafana dashboard templates
+- Support for file-based and inline JSON dashboard registration
+- "Extension" badge for third-party dashboards in admin UI
+- Plugin attribution display for third-party dashboards
+- Security: Path traversal protection for registered dashboard files
+- Isolated mode support for dashboard registration hook
+
+### Changed
+
+- DashboardProvider now supports both built-in and third-party registered dashboards
+- Dashboard cards show source (built-in vs extension) with visual distinction
+
 ## [0.4.5] - 2026-02-02
 
 ### Fixed