Add more guidelines for code review bot (flutter#185367)

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Add guideline for common checklist for code review

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [AI contribution guidelines] and understand my
responsibilities, or I am not using AI tools.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

If this change needs to override an active code freeze, provide a
comment explaining why. The code freeze workflow can be overridden by
code reviewers. See pinned issues for any active code freezes with
guidance.

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[AI contribution guidelines]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#ai-contribution-guidelines
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: chunhtai

.gemini/styleguide.md

@@ -20,9 +20,14 @@ flutter/flutter repository. It is based on the more comprehensive official
   [Effective Dart: Style](https://dart.dev/effective-dart/style)
   should only be followed when it does not conflict with the former.
 
+
 ## Review Agent Guidelines
 
 - Only review changes to the `master` branch. Other changes have already been reviewed (and are being cherrypicked).
+- **Check for potential regressions**: Look for changes that might break existing functionality or introduce unexpected behavior in related areas.
+- **Verify test validity**: Confirm that new or modified tests effectively catch the issue being fixed and would fail if the fix were reverted.
+- **Search for counter-examples**: Identify scenarios or edge cases that the proposed code does not handle. If a counter-example is found, propose a test case to demonstrate the gap.
+- **Suggest simplification and refactoring**: Assess whether the code can be made simpler or refactored to enhance readability and maintainability.
 
 ## General Philosophy