feat(matrixbot): remove the auto-invite welcome flow entirely

Joining the Lotus Guild Space (join_rule: public) triggered a welcome DM;
reacting to it made the bot invite that user into a fixed room list. Now
that #general is published to the public room directory, any stranger can
join the Space and trigger this.

The room list excluded the invite-only rooms, but it included Voice, whose
join rule is `knock`. An invite bypasses a knock gate, so a stranger could
skip the approval step that rule exists to enforce.

Removes welcome.py, the Space-join watcher in Callbacks.member, both
welcome-reaction hooks, and the admin `cleanwelcome` command. The bot no
longer issues invites automatically anywhere. The PL50+ `invite` and
`inviteall` commands are untouched: those are deliberate admin actions.

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

matrixbot/bot.py

@@ -27,7 +27,6 @@ from config import (
 )
 from callbacks import Callbacks
 from utils import setup_logging
-from welcome import log_ready as _welcome_log_ready
 
 logger = setup_logging(LOG_LEVEL)
 
@@ -188,8 +187,6 @@ async def main():
     # Trust devices after initial sync loads the device store
     await trust_devices(client)
 
-    _welcome_log_ready()
-
     logger.info("Bot ready as %s — listening for commands", MATRIX_USER_ID)
 
     # Run sync_forever in a task so we can cancel on shutdown