feat: Mattermost plugin + daemon integration (Phases 2-5)

Plugin (Go server + React webapp):
- Custom post type 'custom_livestatus' with terminal-style rendering
- WebSocket broadcasts for real-time updates (no PUT, no '(edited)')
- KV store for session persistence across reconnects
- Shared secret auth for daemon-to-plugin communication
- Auto-scroll terminal with user scroll override
- Collapsible sub-agent sections
- Theme-compatible CSS (light/dark)

Daemon integration:
- PluginClient for structured data push to plugin
- Auto-detection: GET /health on startup + periodic re-check
- Graceful fallback: if plugin unavailable, uses REST API (PUT)
- Per-session mode tracking: sessions created via plugin stay on plugin
- Mid-session fallback: if plugin update fails, auto-switch to REST

Plugin deployed and active on Mattermost v11.4.0.

.gitignore

@@ -1,3 +1,5 @@
 node_modules/
 coverage/
 *.log
+plugin/server/dist/
+plugin/webapp/node_modules/