Add AGENTS with testing instructions

[pykello]

Summary

• document contribution rules in new AGENTS.md

Testing

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• cargo test --features disable-isal-crypto

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https://chatgpt.com/codex/tasks/task_e_68895123bef48327ab1f0b390aa36811

[greptile-apps]

Greptile Summary

This PR adds a new AGENTS.md file that establishes contribution guidelines for the ubiblk repository. The file provides a simple three-step checklist for contributors to follow before submitting changes:

1. Code formatting validation using cargo fmt --all -- --check
2. Linting with cargo clippy --all-targets --all-features -- -D warnings
3. Comprehensive testing including both standard tests (cargo test) and tests with the optional isa-l_crypto dependency disabled (cargo test --features disable-isal-crypto)

This documentation directly mirrors the automated checks performed in the existing GitHub Actions workflow (.github/workflows/release.yaml), ensuring contributors can locally validate their changes against the same standards enforced by CI. The inclusion of the disable-isal-crypto feature testing aligns with the project's Cargo.toml configuration, which defines this as an optional feature that disables AES and XTS-mode encryption dependencies. This creates a standardized development workflow that helps prevent CI failures and streamlines the contribution process.

Confidence score: 5/5

• This PR is extremely safe to merge as it only adds documentation without modifying any code functionality.
• The commands documented are already established practices in the CI workflow, making this purely a documentation improvement.
• No files need additional attention - this is a straightforward documentation addition.

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