docs: fix reference sidebar ordering and group the subsystem pages

The VitePress config declared no position for the subsystem or other-interface
sections, so `indexOf` returned -1 and sorted them ahead of every declared
group: the reference landing page's own sidebar entry sat 1549px below the
fold. Four subsystem pages also shared `order` values with pages in the same
section, resolved only by sort stability and array concatenation order.

Section placement and collapse move into the manifest as a per-locale
declaration, and `sectionSpec` throws for an undeclared section instead of
sorting it silently to the top. Subsystem pages are grouped by concern, the
six topical groups collapse until one holds the page being read, and page
order derives from array position.

The projector drops the language-switcher line and repository badge the
canonical pages carry for their GitHub readers. The navigation bar gains the
DeepSeek wordmark, a release-stage tag, and a favicon; the sidebar scrollbar
rests invisible and appears while scrolling. Subsystem pages carry a two-level
outline, and the two plugin-development tracks now cross-link.

docs/cordis-tutorial/index.md

@@ -8,6 +8,8 @@ The audience is agent developers. You do not need deep TypeScript experience; th
 
 If you want the condensed concept reference instead of a walkthrough, read the [Cordis primer](../cordis-primer.md). The exhaustive API reference lives in the generated `cordis-surface` regions on the [subsystem pages](../subsystems/core.md) and the [Cordis core API](../cordis-api/context.md) pages.
 
+To write plugins for the harness itself — loaded from a `cordis.yml` and driven from the Web UI rather than the launcher below — start from [your first Harness plugin](../user/develop/basic/index.md).
+
 ## Setup
 
 You need a clone of this repository with dependencies installed; the [development guide](../development.md#setup-tutorial) lists the prerequisites. No API key is needed for this tutorial; every example runs keylessly.