docs: update AGENTS.md with CI guidance and change-context table

[bearomorphism]

Add practical CI validation commands, pipeline structure, common failure patterns, and known type system limitations learned from real PR workflows.

Description

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

Steps to Test This Pull Request

Additional Context

[Lee-W]

But we're to change to ty in #1932, or are we going to wait for that?

[bearomorphism]

sounds good, thanks