docs(desktop): document the desktop plugin SDK (@hermes/plugin-sdk) (#67301)

Add an end-to-end developer guide for extending the native Hermes Desktop
app introduced in #60638: the HermesPlugin contract, PluginContext, every
contribution area (panes, routes, sidebar nav, status/title bar, palette,
keybinds, themes, composer, mount-scoped Contribute), the host API, the
React Query + nanostores data layer, the UI kit + theme variables, the
scoped ctx.rest/ctx.socket backend (plugin_api.py under /api/plugins/<id>)
and its separate enable gate, Settings/defaultEnabled/storage, bundled
plugins, the security model, pitfalls, and a full reference.

- Register the page in the sidebar under Extending -> Plugins.
- Disambiguate from the unrelated web-dashboard plugin SDK from both
  directions, and add a map-table row + a desktop user-guide pointer.
- Fill the gaps in the agent-facing hermes-desktop-plugins skill
  (ctx.rest/socket + backend, React Query, defaultEnabled, Contribute)
  and point it at the new reference so agents know the SDK when writing
  addons.

website/docs/developer-guide/plugins/index.md

@@ -15,6 +15,8 @@ Hermes has several distinct pluggable interfaces — some use Python `register_*
 | If you want to add… | Read |
 |---|---|
 | Custom tools, hooks, slash commands, skills, or CLI subcommands | **This guide** (the general plugin surface) |
+| A **native desktop app** extension (panes, pages, status bar, palette, themes) | [Desktop Plugin SDK](/developer-guide/desktop-plugin-sdk) |
+| A **web dashboard** extension (tabs, shell slots, themes) | [Extending the Dashboard](/user-guide/features/extending-the-dashboard) |
 | An **LLM / inference backend** (new provider) | [Model Provider Plugins](/developer-guide/model-provider-plugin) |
 | A **gateway channel** (Discord/Telegram/IRC/Teams/etc.) | [Adding Platform Adapters](/developer-guide/adding-platform-adapters) |
 | A **memory backend** (Honcho/Mem0/Supermemory/etc.) | [Memory Provider Plugins](/developer-guide/memory-provider-plugin) |