Add AGENTS.md files to provide guidelines for project and packages

[gerteck]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:

Add root AGENT.md in root of directory and root of each package

Related to #2812

Anything you'd like to highlight/discuss:

Referred to some sources:

• https://www.aihero.dev/a-complete-guide-to-agents-md
• https://medium.com/google-cloud/starter-gemini-md-file-within-a-project-directory-for-gemini-cli-cb8ba0f372cb
• https://www.builder.io/blog/agents-md
• https://agents.md/

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)

Add AGENTS.md files to root and package roots

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

---

Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.85%. Comparing base (58f396f) to head (cf34991).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2814   +/-   ##
=======================================
  Coverage   71.85%   71.85%           
=======================================
  Files         134      134           
  Lines        7340     7340           
  Branches     1563     1620   +57     
=======================================
  Hits         5274     5274           
+ Misses       2020     1938   -82     
- Partials       46      128   +82     

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[yihao03]

Great! just a few minor comments

[Harjun751]

Other than what Yi Hao flagged, LGTM. I think I'd be a good robot if I followed this.

[Incogdino]

LGTM