feat: optional shared/external MemPalace via MEMPALACE_REMOTE_URL

Wire the shared-palace option (implemented in mempalace-toolkit's mempalace.ts)
into the container:
- .env.example: document MEMPALACE_REMOTE_URL / MEMPALACE_REMOTE_TOKEN (env_file-only,
  per this repo's convention).
- docker-compose.mempalace.yml: optional shared server (mempalace-mcp --transport http),
  loopback-bound by default.
- docker-compose.yml: local-vs-external note on the palace-volume comment.
- README + CHANGELOG (Unreleased).

README.md

@@ -29,7 +29,10 @@ on the host.
 - ChromaDB embedding model pre-warmed at build time (`all-MiniLM-L6-v2`)
 
 The host-mounted palace at `~/.mempalace` is shared across the host and
-this container so all your agents share one brain.
+this container so all your agents share one brain. To instead share a palace
+across *several* containers/harnesses, set `MEMPALACE_REMOTE_URL` to a shared
+MemPalace HTTP endpoint (see `.env.example` and `docker-compose.mempalace.yml`);
+the bridge then connects over HTTP and spawns no local server.
 
 ### Modern CLI tooling