docs(collab): clarified relay distribution status

Documented that the hosted Go relay has no published source or standalone binaries, and linked the repository's WebSocket-only local protocol relay with its startup command and limitations.

Fixes #8165

docs/collab.md

@@ -112,6 +112,10 @@ Set `collab.webUrl` when the browser UI is hosted separately from the websocket
 
 ## Self-hosting the relay
 
+The production relay is not currently distributed for self-hosting: its Go source and standalone binaries are not published. The endpoint list below documents the hosted service's network contract, not an installable release.
+
+For local protocol development, this repository includes a source-available, WebSocket-only stand-in at [`packages/collab-web/scripts/local-relay.ts`](../packages/collab-web/scripts/local-relay.ts). Run `bun run relay` from `packages/collab-web` to listen on `ws://localhost:7466`. It implements `/r/<roomId>` but does not serve the browser client, `/share` blobs, or `/healthz`, so it is not a replacement for the production service.
+
 The relay is a small content-blind Go service. It keeps no state beyond live connections and exposes:
 
 - `GET /` — the static collab-web guest client (target of the `/collab` deep link),