Harden /ask helper, UDP replies and run_lua in multiplayer

- Pass the /ask question to claude on stdin so it can't be parsed as a CLI option.
- Block run_lua in multiplayer unless an admin enables the new
  "Allow Lua queries in multiplayer" map setting.
- Ignore UDP packets that don't come from the game's port, in both the
  MCP server and the ask helper.
- Build the screenshot path from our own name instead of the game's reply,
  and restrict screenshot names in the mod.
- Validate ask packets, cap question length and limit concurrent claude runs.
- Replace hard-coded home paths in the README and systemd unit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/claude_co_engineer/server.py

@@ -268,7 +268,9 @@ async def screenshot(
     meta = await game.request(
         "screenshot", x=x, y=y, width_tiles=width_tiles, alt_mode=alt_mode, daylight=daylight, surface=surface, name=name
     )
-    path = SCRIPT_OUTPUT / meta.pop("path")
+    # Built from our own name rather than the reply, so a reply can't point the unlink below elsewhere.
+    meta.pop("path", None)
+    path = SCRIPT_OUTPUT / "claude-co-engineer" / f"{name}.jpg"
     try:
         data = await _read_finished_jpeg(path)
     finally: