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).

CHANGELOG.md

@@ -15,6 +15,16 @@ Pre-v1.0.0 tags followed the pi npm version (`v{pi_version}[letter]`).
 
 ### Added
 
+- **Share one MemPalace across containers via `MEMPALACE_REMOTE_URL`.** The
+  `mempalace.ts` bridge (from `mempalace-toolkit`) can now connect to a shared
+  MemPalace over HTTP instead of spawning a per-container local server: set
+  `MEMPALACE_REMOTE_URL=http://<host>:8765/mcp` (optionally
+  `MEMPALACE_REMOTE_TOKEN`) in `.env` and no local `mempalace-mcp` is spawned.
+  A new `docker-compose.mempalace.yml` stands up such a shared server
+  (`mempalace-mcp --transport http`). Leaving the URL unset keeps the default
+  local-per-container palace. See `.env.example`. (The HTTP transport is
+  unauthenticated — keep it on a trusted network or behind a reverse proxy.)
+
 - **Two non-modal terminal editors alongside `nvim`: `nano` and `micro`.**
   The image previously shipped only `nvim` (with `EDITOR=nvim`), a modal
   vi-style editor. Not everyone is comfortable with vi keybindings, so both