feat: optional shared/external MemPalace + fix validate.yml false-red on base changes

Added:
- generate-config.py registers mempalace as a remote MCP endpoint when
  MEMPALACE_REMOTE_URL is set (MEMPALACE_REMOTE_TOKEN -> Bearer), else the local
  stdio command as before. Same env contract as pi-devbox's bridge.
- docker-compose.mempalace.yml: optional shared server (mempalace-mcp --transport http),
  loopback-bound by default.
- compose (both) + .env.example(.shared): MEMPALACE_REMOTE_URL/TOKEN + local-vs-external docs.

Fixed:
- validate.yml: skip validate-base/validate-omos on base-changing commits
  (base-change-warning now exports base_changed). Previously a commit that changed
  the base AND tightened smoke-test.sh in lockstep (v2.4.0 nano/micro) hard-failed
  against the stale base-latest until the release rebuilt it. actionlint clean.

README + CHANGELOG (Unreleased).

docker-compose.shared.yml

@@ -38,6 +38,10 @@ services:
       - TERM=xterm-256color
       - GITEA_ACCESS_TOKEN=${GITEA_ACCESS_TOKEN:-}
       - GITEA_HOST=${GITEA_HOST:-}
+      # MemPalace transport: empty = LOCAL per-container palace (default);
+      # set MEMPALACE_REMOTE_URL to share one external palace. See .env.example.
+      - MEMPALACE_REMOTE_URL=${MEMPALACE_REMOTE_URL:-}
+      - MEMPALACE_REMOTE_TOKEN=${MEMPALACE_REMOTE_TOKEN:-}
     volumes:
       # Host workspace — user's project directory
       - ${WORKSPACE_PATH:-~/src}:/workspace
@@ -73,7 +77,9 @@ services:
       # Persist uv data (Python installs)
       - devbox-uv:/home/developer/.local/share/uv
 
-      # Optional: persist MemPalace data (conversation memory, knowledge graph)
+      # Optional: persist MemPalace data (conversation memory, knowledge graph).
+      # LOCAL palace only (default). In EXTERNAL mode (MEMPALACE_REMOTE_URL set)
+      # the shared server owns the data, so this volume is irrelevant.
       # - devbox-palace:/home/developer/.mempalace
 
       # Optional: persist ChromaDB embedding model cache (~79 MB)