fix(matrixbot): replace mcrcon with a thread-safe RCON client

mcrcon implements its read timeout with signal.SIGALRM, which only works
on the main thread. The Minecraft commands call it from a worker thread
via loop.run_in_executor, so it raised "signal only works in main thread
of the main interpreter" on every invocation. The replacement uses
socket.settimeout(), which has no such restriction.

This code was already deployed on LXC 151 but had never been committed,
so any matrixbot deploy would have reverted it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

matrixbot/requirements.txt

@@ -2,4 +2,3 @@ matrix-nio[e2e]
 python-dotenv>=1.2.2
 aiohttp
 markdown
-mcrcon