exclude AGENTS.md in MANIFEST.in

[chrishavlin]

after merging #5546 , uvx check-manifest -vvv started failing, this fixes that. ( https://github.com/yt-project/yt/actions/runs/37983468943/job/113999496582 ).

AI Disclosure

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I used AI for (leaving both boxes unchecked certifies that no AI was used in these changes):

• low level assistance: code-completion, syntax lookup, explaining existing code.
• substantive generation: substantive portions of code and/or documentation were written by AI then edited or submitted as is. This includes copying code from an AI chat or agentic coding. I have read and can explain every changed line.

I used the following AI tools:

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.
• AI Disclosure completed if AI tools were used.