Adds AGENTS.md.

Establishes some rules that we would like AI agents to follow when creating issues or pull requests.

Author: schauder

AGENTS.md

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+## Issues
+
+- For non-trivial issues, include a link to a sample project that reproduces the issue.
+- Assume all versions to be that of the latest release. Only specify versions that deviate from that.
+- Specify dependencies only if the problem exists for one version or variant, but not for others.
+- Don't repeat information specified in the reference reproducer.
+- Add `Issue created by AI` to the issue description.
+
+## Pull requests
+
+- If there is an issue that gets fixed by this PR, add the keyword `Closes #<issue number>` to the PR description.
+- Add `PR created by AI` to the PR description.
+- If the change is not trivial, make sure there exists an issue for it.
+- Make the description short. 
+- Do not repeat information. 
+- Avoid superfluous embellishments. 
+- Use formatting sparingly. 
+- Do not repeat information contained in the issue description.
+- Make sure the PR builds successfully, including tests.
+- Make sure the PR contains tests for changed behavior.
+- Make sure the PR adheres to the contribution guidelines: https://github.com/spring-projects/spring-data-build/blob/main/CONTRIBUTING.adoc.