Try and discourage use of `git push --force`

### What is the problem the feature request solves?

I would like to update the contributor guide and `AGENTS.md` to try and discourage use of `git push --force` and instead use `--force-with-lease`. This would prevent accidentally deleting commits from other maintainers in rare cases where multiple maintainers are pushing to the same PR.

### Describe the potential solution

_No response_

### Additional context

_No response_

Review the contributor guide and AGENTS.md for guidance on force-pushing. Update both documents to discourage git push --force and recommend --force-with-lease instead, then verify that the guidance is consistent and clearly explains the safer workflow.

Written by the indexing model from the issue text.