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.

plugin/assets/icon.svg

@@ -0,0 +1,3 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="#166de0">
+  <path d="M20 4H4c-1.1 0-2 .9-2 2v12c0 1.1.9 2 2 2h16c1.1 0 2-.9 2-2V6c0-1.1-.9-2-2-2zm-1 14H5V6h14v12zM7 9h2v2H7zm0 4h2v2H7zm4-4h6v2h-6zm0 4h6v2h-6z"/>
+</svg>