Add MemPalace local-first AI memory system to base image

Install mempalace via pip in the Dockerfile. Provides 29 MCP tools
for semantic search over conversation history, knowledge graph
queries, agent diaries, and wing/room/drawer management. Everything
runs locally — no API keys, no data egress.

Integration:
- Dockerfile: pip install mempalace (with --break-system-packages
  for Debian trixie PEP 668 compliance)
- entrypoint-user.sh: auto-initializes palace for /workspace on
  first run (idempotent, skips if palace exists)
- entrypoint.sh: adds ~/.mempalace to the volume ownership-fix loop
- docker-compose.yml + shared: optional devbox-palace named volume
  at ~/.mempalace (commented out by default — user opts in)

Users configure MCP integration by adding a mempalace server entry
to their opencode.json. No wrapper plugin needed — the upstream
Python MCP server is used directly.

Docs updated: README.md (new MemPalace section with setup, MCP
config, usage examples, storage details), DOCKER_HUB.md (data
storage table + tools list), CHANGELOG.md (unreleased entry).

docker-compose.shared.yml

@@ -61,6 +61,9 @@ services:
       # Persist uv data (Python installs)
       - devbox-uv:/home/developer/.local/share/uv
 
+      # Optional: persist MemPalace data (conversation memory, knowledge graph)
+      # - devbox-palace:/home/developer/.mempalace
+
       # Optional: AWS credentials (per-user if available)
       # - ${HOME}/${SIGNUM}/.aws:/home/developer/.aws
 
@@ -70,3 +73,4 @@ volumes:
   devbox-zoxide:
   devbox-nvim-data:
   devbox-uv:
+  # devbox-palace: