Fix ruff lint errors across matrixbot (F401, F841, E402)

Remove unused imports: logging from bot.py and config.py, RoomMessageText/
UnknownEvent from callbacks.py, functools.partial and MAX_INPUT_LENGTH from
commands.py. Rename unused local variables to _ (resp in cmd_ping, symbols in
render_keyboard_plain, guesses_left in two wordle functions). Move wordle import
to top of commands.py to fix E402.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

matrixbot/callbacks.py

@@ -1,7 +1,7 @@
 import logging
 from functools import wraps
 
-from nio import AsyncClient, RoomMessageText, UnknownEvent
+from nio import AsyncClient
 
 from config import BOT_PREFIX, MATRIX_USER_ID
 from commands import COMMANDS, metrics