Contributing guide should note that PRs must include the template checklist

[jason-curtis]

While working on #2705, a coding agent created a PR using gh pr create --body "..." which overrode the default PR template, omitting the checklist. The contributing guide should mention that all PR descriptions must include the PR checklist, especially when using workflows that override the default template. This applies to coding agents and various other tools that may not respect the template (at one point VS Code had this issue; not sure if it's still the case).

This will make sure coding agents and regular users know to include the template contents.