Add AI tooling disclosure text to contributor guide and fields to PR templates

### Is your feature request related to a problem or challenge?

Inspired by https://github.com/ghostty-org/ghostty/pull/8289 I think it might be a good idea to add some text to the contributor guide requesting usage of AI tooling to be disclosed. Additionally, adding fields to PR templates encourages people to fill in this information proactively.

Use of AI tools should not be discouraged, but as a courtesy to the human being reviewing changes it would be nice if any output from an AI system is marked as such. The referenced example explains this much better than I can.

### Describe the solution you'd like

See https://github.com/ghostty-org/ghostty/blob/main/CONTRIBUTING.md#ai-assistance-notice for a possible source of inspiration.

### Describe alternatives you've considered

#### Do nothing.

I'm not sure to what extent AI slop PRs are already a problem in this project. Maybe adding the PR template fields is too much of a nuisance for regular contributors.

### Additional context

_No response_

Start with CONTRIBUTING.md and the repository’s existing pull request templates, then compare the linked Ghostty AI assistance notice for wording and scope. Done means the contributor guide explains how to disclose AI tooling and the relevant PR templates include fields prompting that disclosure.

Written by the indexing model from the issue text.