Updating username could lead to getting around being `denounced`

A good point was brought up when we were thinking through implementing the vouch system for the OpenCode repo (we see a lot of AI slop, especially in issues) and someone in https://github.com/anomalyco/opencode/pull/12640#issuecomment-3865765666 brought up the thought of if someone changes their username they would be able to get around being denounced.

A bad user (that wants to spam) might change their username to get around being "banned/denounced" from a repo.

I think a way to solve this would be to have in the .td file would be to include the userid of the user as well so that:

-github:slopmaster3000 Submitted endless amounts of AI slop

would become

-github:slopmaster3000:128301923810298 Submitted endless amounts of AI slop

The only caveat to this is since this isn't Github specific (and I don't have experience with other providers like Gitlab) I can't confirm if this is a reliable way to attain user-ids.

Start by tracing how .td entries are parsed and how provider user identities are resolved for denouncements. Check the supported provider integrations before choosing an identity field; done means a username change cannot bypass an existing denouncement without breaking providers that lack stable user IDs.