Review findings on #120922.
- Hiding `capabilities` removed the row that hosts the Plugins tab, the
user's only path to a plugin's own off-switch — the design law forbids a
plugin locking the user out of disabling it. `NEVER_HIDDEN` keeps that
row; it can still be re-ordered.
- Docs said "first-registered order wins", but `registry.getArea` sorts by
`Contribution.order` then insertion, so a later plugin with `order: -1`
pre-empts. The registry exposes no insertion order, so the docs (and the
SDK/store comments) now state the real rule: lowest `order`, then
registration. A registry-backed test pins it.
- Docs said a contributed row's id equals its `SIDEBAR_NAV_AREA` id, but
`createPluginContext.scope` namespaces it to `${pluginId}:${id}`; the
docs now give the namespaced form to use in `hide`/`order`.
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