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).

.env.example

@@ -9,6 +9,19 @@ WORKSPACE_PATH=~/projects
 # Path to SSH keys on host
 SSH_KEY_PATH=~/.ssh
 
+# ── MemPalace memory (local by default) ───────────────────────────
+# By default the mempalace.ts extension spawns a LOCAL mempalace-mcp stdio
+# server (palace at ~/.mempalace). Uncomment the devbox-palace volume in
+# docker-compose.yml to persist it across container recreation.
+#
+# To instead share ONE MemPalace across containers/harnesses (pi + opencode
+# + native), set the URL below. When set, the extension connects over HTTP
+# and NO local mempalace-mcp is spawned; the devbox-palace volume is then
+# irrelevant. MEMPALACE_REMOTE_TOKEN, if set, is sent as a bearer token.
+#   Serve it with:  mempalace-mcp --transport http --host 0.0.0.0 --port 8765
+# MEMPALACE_REMOTE_URL=http://mempalace.lan:8765/mcp
+# MEMPALACE_REMOTE_TOKEN=
+
 # ── LAN access from the container (host-OS-agnostic) ─────────────────
 # On VM-backed hosts (macOS OrbStack / Docker Desktop) the container can't
 # reach the host's directly-attached LAN peers by default. The entrypoint