filters plugin uses msg.frm.room which does not exist for DMs on Zulip

7dcc1e07 introduces filters. On Zulip, in direct messages, this causes

```
Traceback (most recent call last):
File "/.../errbot/errbot/core.py", line 345, in _process_command_filters
msg, cmd, args = cmd_filter(msg, cmd, args, dry_run)
File "/.../corobo/utils/filters.py", line 21, in filters
if cmd in commands and msg.frm.room.uri == room:
AttributeError: 'ZulipPerson' object has no attribute 'room'
```

Start in corobo/utils/filters.py at line 21 and trace how _process_command_filters in errbot/core.py passes direct messages into the filter. Check the ZulipPerson message shape and existing room-filter behavior. Done means direct messages no longer raise AttributeError while room-based filtering continues to work.

Written by the indexing model from the issue text.