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).

docker-compose.mempalace.yml

@@ -0,0 +1,84 @@
+# Shared MemPalace server (optional) — one palace for many clients.
+#
+# Runs `mempalace-mcp` over HTTP so several containers/harnesses (pi +
+# opencode + native) can share ONE palace instead of each keeping its own.
+# Point every client at it by setting, in that client's .env:
+#
+#   MEMPALACE_REMOTE_URL=http://<reachable-host>:8765/mcp
+#
+# (see .env.example). When set, the client connects over HTTP and does NOT
+# spawn its own local mempalace-mcp.
+#
+# Start:   docker compose -f docker-compose.mempalace.yml up -d
+# Stop:    docker compose -f docker-compose.mempalace.yml down
+# Logs:    docker compose -f docker-compose.mempalace.yml logs -f
+#
+# Why reuse the devbox image? mempalace-mcp is already installed in it, and
+# reusing it GUARANTEES the server's mempalace version matches the clients'
+# (both are pinned by the same image build). Override with a slimmer image via
+# MEMPALACE_SERVER_IMAGE if you prefer (it must provide `mempalace-mcp`).
+#
+# ⚠ SECURITY: mempalace-mcp's HTTP transport has NO authentication of its own.
+# Do NOT expose port 8765 to an untrusted network. The default below binds to
+# 127.0.0.1 (host loopback) only. To let sibling containers reach it, either
+# attach them to the shared `mempalace-net` network (container-to-container, no
+# host port needed — use http://mempalace-server:8765/mcp), or front it with a
+# reverse proxy that enforces MEMPALACE_REMOTE_TOKEN as `Authorization: Bearer`.
+
+name: mempalace-server
+
+services:
+  mempalace:
+    image: ${MEMPALACE_SERVER_IMAGE:-joakimp/pi-devbox:latest}
+    container_name: mempalace-server
+    # Bypass the devbox entrypoint (dev-shell/LAN/config setup) and run the
+    # HTTP MCP server directly. HOME + explicit --palace pin the data path so
+    # it does not depend on the image's default user/HOME. Runs as root so it
+    # can initialise the fresh named volume; the volume is dedicated to this
+    # server (clients reach it over HTTP, never by mounting it).
+    entrypoint: []
+    user: "0:0"
+    environment:
+      - HOME=/data
+    command:
+      - mempalace-mcp
+      - --transport
+      - http
+      - --host
+      - "0.0.0.0"
+      - --port
+      - "8765"
+      - --palace
+      - /data/.mempalace
+    restart: unless-stopped
+    # Loopback-only by default (see SECURITY note). Use "8765:8765" to expose on
+    # all host interfaces, or drop `ports:` entirely and rely on mempalace-net.
+    ports:
+      - "127.0.0.1:8765:8765"
+    volumes:
+      # The shared palace data — precious; back this up.
+      - mempalace-shared:/data/.mempalace
+      # Embedding-model cache (~79 MB, disposable) so search does not re-download.
+      - mempalace-shared-chroma:/data/.cache/chroma
+    networks:
+      - mempalace-net
+    healthcheck:
+      # A tools/list round-trip proves the server is answering MCP (python3 is
+      # always present — mempalace itself is a python tool in the image).
+      test:
+        - CMD
+        - python3
+        - -c
+        - "import urllib.request,json; d=json.dumps({'jsonrpc':'2.0','id':1,'method':'tools/list','params':{}}).encode(); r=urllib.request.Request('http://127.0.0.1:8765/mcp',data=d,headers={'Content-Type':'application/json','Accept':'application/json'}); urllib.request.urlopen(r,timeout=5).read()"
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 60s
+
+volumes:
+  mempalace-shared:
+  mempalace-shared-chroma:
+
+networks:
+  mempalace-net:
+    name: mempalace-net