update and (re)enable assisted code reviews (#8816)

Updates config to be inline with flutter's (except where noted) and
enables reviews on pull requests.

---

A sample review provoked by a bogus `TODO` and system out.

<img width="590" height="616" alt="image"
src="https://github.com/user-attachments/assets/69e96f1a-9ac8-4afb-81e0-83d1d4eb6c4b"
/>

Note how it's reading from our style guide and recommending our bespoke
logger.

Closes: #8815.



---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md](https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Dart contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin
best practices
([discussion](#8098)).
</details>

Author: pq

.gemini/config.yaml

@@ -1,13 +1,25 @@
-# These are all defaults for now. Descriptions for these settings: https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#config.yaml-schema
+# Based on Flutter config: https://github.com/flutter/flutter/blob/master/.gemini/config.yaml
+#
+# Schema: https://developers.google.com/gemini-code-assist/docs/customize-gemini-behavior-github#config.yaml-schema
+
+# No poetry in summaries (DEFAULT).
 have_fun: false
+
 code_review:
+  # (DEFAULT)
   disable: false
+  # LOW, MEDIUM, HIGH or CRITICAL
   comment_severity_threshold: MEDIUM
+  # -1 is unlimited (DEFAULT)
   max_review_comments: -1
   pull_request_opened:
+    # (DEFAULT)
     help: false
+    # These tend to be verbose, and since we expect PR authors to clearly
+    # describe their PRs this would be at best duplicative.
     summary: false
-    code_review: false
+    code_review: true
+    # (DEFAULT) -- Note that this is disabled for flutter
     include_drafts: true
 ignore_patterns: []