docs(maintainers): adding block kit types instructions

[srtaalej]

Summary

Add "Adding a New Block Kit Type" section to AGENTS.md documenting how to define, register, export, and test new Block Kit models (blocks, elements, and composition objects)

Test plan

• Verify AGENTS.md renders correctly on GitHub
• Confirm the documented patterns match actual code in slack_sdk/models/blocks/

[zimeg]

@srtaalej Is this a document we can link to from this root file as:

• AGENTS.md
• slack_sdk/models/AGENTS.md

I'm worried of overloading the initial context when these details might be saved for later?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.01%. Comparing base (9435412) to head (e9a85f5).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## main #1861 +/- ##
=======================================
Coverage 84.01% 84.01% =======================================
Files 117 117 Lines 13256 13256 =======================================
Hits 11137 11137 Misses 2119 2119 

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[zimeg]

@srtaalej Looks solid and I'm optimistic for upcoming additions that use this 🤖

A few notes below are thoughts but we should refine this document with future findings!

[zimeg]

🤖 suggestion(non-blocking): Including a brief description of this entire package might be useful here. I'm not sure how much context otherwise is known but we can iterate on this as we find useful?