Plugin UI Extension Points — pyrite

Allow plugins to extend the web UI:

Backend:

`GET /api/plugins` — list registered plugins with entry types and capabilities

Frontend:

`plugins/registry.ts` — plugin UI extension registry

Plugins can register: sidebar items, command palette actions, entry type renderers

Extension point system for custom entry type views

This bridges the existing backend plugin system with the web frontend.

Plugin UI Extension Pointsbacklog_item