chore: improve AGENTS.md

[WilliamBergamin]

Summary

This PR aims to improve the AGENTS.md based on standards from https://agents.md/ and add some bias towards using buitl in middleware when adding new features

Category

• slack_bolt.App and/or its core components
• slack_bolt.async_app.AsyncApp and/or its core components
• Adapters in slack_bolt.adapter
• Document pages under /docs
• Others

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to those rules.

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run ./scripts/install_all_and_run_tests.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.67%. Comparing base (0fc5380) to head (8097936).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1458   +/-   ##
=======================================
  Coverage   90.67%   90.67%           
=======================================
  Files         226      226           
  Lines        7205     7205           
=======================================
  Hits         6533     6533           
  Misses        672      672           

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