Move the section out of the middle of the host.request explanation to the end
of "Host API" and complete it per the #116305 design: TS signatures; the
arbitration rule (closed allowlist -> owning store setter, sync throws, never
localStorage); the teardown rule (host is a module singleton and cannot
attribute a subscriber, so plugins must ctx.onDispose the subscribe disposer);
the declined keys with the reason each stays out (keybind map -> KEYBINDS_AREA,
theme/mode -> THEMES_AREA, pluginDecisions -> Plugins tab, appearance-hub's
extra keys -> per-store follow-ups); and the exact hermes-appearance-hub /
prompt-snippets migration off localStorage + synthetic StorageEvent.
Part of #116305
21f6e35643