Add AGENTS guidelines

AGENTS.md

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+# AGENTS Instructions for Quizzical Beats
+
+These instructions apply to the entire repository.
+
+## Development Guidelines
+- Follow **PEP 8** with a maximum line length of **100 characters**.
+- Use **4 spaces** for indentation. Tabs are not allowed.
+- Provide docstrings for all functions and classes using Google's style guide.
+- Keep Flask routes organized using blueprints and prefer class-based views for complex endpoints.
+
+## Commit Messages
+- Write clear, concise commit messages in English.
+- Reference related issues when applicable.
+
+## Testing
+- Run `pytest` before committing changes. If tests fail due to missing
+  dependencies or external services, mention this in the PR.
+
+## Pull Request
+- Summarize major changes and reference relevant documentation updates.
+- Include test results in the PR description.
+