Adds a persisted user-level marketplace registry
(~/.cowork/config/marketplaces.json) with the built-in marketplace as
an implicit, non-removable first entry. Plugin and skill catalogs,
available-item offers, plugin detail lookups, and sourceHash-based
update annotation now aggregate across every configured marketplace,
with per-marketplace failure isolation. New JSON-RPC methods
cowork/marketplaces/read, add, and remove return a marketplaces_list
event; add validates the manifest before persisting and both mutations
refresh the remote-inclusive skill and plugin catalogs. Documented in
docs/websocket-protocol.md.
Co-authored-by: Cursor <cursoragent@cursor.com>
