MINOR: Add PR description guidance to AGENTS.md (#23739)

Add a coding convention to AGENTS.md stating that the PR description
becomes the squash-merged commit message. Agents tend to write PR
descriptions aimed at reviewers, listing the tests and checks they ran.
Once merged, these notes stay in the commit history, where they are
noise to future readers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Reviewers: Ken Huang <s7133700@gmail.com>, Chia-Ping Tsai
<chia7712@gmail.com>, TengYao Chi <frankvicky@apache.org>, Eric Chang
<e850506@gmail.com>, Ming-Yen Chung <mingyen066@gmail.com>

AGENTS.md

@@ -128,6 +128,7 @@ Checkstyle reports: `<module>/build/reports/checkstyle/`.
 ## Coding conventions
 
 - **Commits / PR titles**: Start with `KAFKA-XXXXX`, `MINOR`, or `HOTFIX`. Use `KAFKA-XXXXX` only when there is a valid Jira ticket for the change.
+- **PR descriptions**: The PR title and description become the squash-merged commit message. In the description, explain what the change does and why it is needed, if not obvious. Keep it short for small changes. Put any notes for reviewers, such as the tests you ran, in a separate comment.
 - **AI-generated contributions**: Follow the [AI-Generated Contributions](CONTRIBUTING.md#ai-generated-contributions) section in `CONTRIBUTING.md` — add a `Co-Authored-By` or `Generated-by` commit trailer for AI-assisted changes.
 - **Public API / KIP**: Changes to public interfaces, wire protocol, configurations, or metrics generally require a [KIP](https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Improvement+Proposals). See the `javadoc` `include` list in [build.gradle](build.gradle) for packages treated as public API.
 - **License header**: New source files need the standard ASF license header (see existing files in the same module).