Add review agent style guidelines to .gemini/styleguide.md (flutter#174000)

A copy of @stuartmorgan-g change at
flutter/packages#9805

> Initial attempt at adding guidance for GCAfGH to try cut down on the
behavior observed so far that it:
> - Almost always includes generic praise for the PR, which is unhelpful
since the agent has no context for evaluating what PRs we actually want
(see [this particularly problematic
example](flutter/packages#9795 (review)))
> - Appears to trust PR descriptions. E.g., recently praising a PR that
was missing tests of most of the PR, but had a PR description saying it
included extensive testing, as being extremely well tested.

Here are examples of summaries from the packages repo before and after
this change:

Before (praise like "great refactoring effort", "significantly improves
the modularity and maintainability", "extensive documentation updates
are also a major improvement"):
> This pull request is a great refactoring effort. Moving the ProxyAPI
generation logic into a dedicated helper file
(`proxy_api_generator_helper.dart`) significantly improves the
modularity and maintainability of the Dart generator. The extensive
documentation updates are also a major improvement, making the concepts
around ProxyAPIs much clearer for developers.

flutter/packages#9756



After (drier, describing the actual change without making normative
evaluation):
> This pull request introduces a new `clearAuthorizationToken` method
across all layers of the `google_sign_in` plugin. This allows developers
to manually clear a cached authorization token if it becomes invalid.
The implementation is handled natively on Android, is a no-op on iOS (as
the SDK handles it), and clears a local cache on the web. The changes
include updates to the platform interface, implementations for each
platform, and corresponding tests and documentation.


flutter/packages#9846 (review)

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

**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
[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: jmagman

.gemini/styleguide.md

@@ -50,6 +50,16 @@ flutter/flutter repository. It is based on the more comprehensive official
 - **Provide illustrations or screenshots** for widgets.
 - Use `///` for public-quality documentation, even on private members.
 
+## Review Agent Guidelines
+
+When providing a summary, the review agent must adhere to the following principles:
+- **Be Objective:** Focus on a neutral, descriptive summary of the changes. Avoid subjective value judgments
+  like "good," "bad," "positive," or "negative." The goal is to report what the code does, not to evaluate it.
+- **Use Code as the Source of Truth:** Base all summaries on the code diff. Do not trust or rephrase the PR
+  description, which may be outdated or inaccurate. A summary must reflect the actual changes in the code.
+- **Be Concise:** Generate summaries that are brief and to the point. Focus on the most significant changes,
+  and avoid unnecessary details or verbose explanations. This ensures the feedback is easy to scan and understand.
+
 ## Further Reading
 
 For more detailed guidance, refer to the following documents: