feat: v3.4.0 — OpenPanel plugin public release (42 tools)

OpenPanel product analytics plugin fully reviewed, tested, and published.
Works with both self-hosted and cloud (openpanel.dev) instances.

- 42 tools: event tracking, data export, analytics, project/client management
- All tools use public REST APIs (Track, Export, Insights, Manage)
- Client modes: write (tracking), read (analytics), root (full access)
- Service page with description, setup notes, WordPress plugin download
- Dynamic URL hints in Add Site form
- 62 unit tests
- ENABLED_PLUGINS default now includes openpanel

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

env.example

@@ -32,8 +32,8 @@ DASHBOARD_SESSION_SECRET=
 # ============================================
 # Comma-separated list of plugins visible to public (OAuth) users.
 # Admin users always see all plugins regardless of this setting.
-# Default (if not set): wordpress,woocommerce,supabase
-# ENABLED_PLUGINS=wordpress,woocommerce,supabase
+# Default (if not set): wordpress,woocommerce,supabase,openpanel
+# ENABLED_PLUGINS=wordpress,woocommerce,supabase,openpanel
 
 # ============================================
 # ADMIN SYSTEM (Track F.4)