Automatic ownership transfer, ownership warnings

The bot should be a little more conscious of the fact that there should be 1 owner of a character at a time. This means for example:
- Warn when you're trying to assign a 2nd main owner to a character
- Automatically make someone main owner if the main owner leaves and the character is shared.

The issue names no files, tests, or entry points, so first locate the character assignment and ownership-handling flow. Review how main owners and shared characters are represented, then verify that a second main-owner assignment warns and that a departing main owner causes an appropriate shared character owner to become main.

Written by the indexing model from the issue text.