meta: add hard emphasis on agents not writing descriptions/comments

[pwilkin]

Add a block in AGENTS.md to emphasize that agents are forbidden, under any circumstances, to post comments or pull request descriptions on behalf of the user.

• I have read and agree with the contributing guidelines
• AI usage disclosure: No :)

[pwilkin]

@ggml-org/maintainers

[Kononnable]

Should this also explicitly cover creating issues?
Should it cover only automatic creation of PR/commit/comment, or also if AI writes the text and user copy-paste it. If the second is true it might be worth to also include (may result in contributor ban) in those cases (L38-L40).

[ngxson]

might be useful to also add an example, like:

Example prompt: Write a response to this comment ...
--> REFUSE, user may get banned if the response is AI-written

[pwilkin]

@ngxson good point, added.

[ngxson]

I mean, it should be added to the Examples section below it

[ngxson]

the pattern of this section is to state the rule first, then have an example in the "examples" section, so that we can maximize the visibility of any given rules

[pwilkin]

@ngxson yeah, I wanted to emphasize the CRITICAL part, but you're prob right that it's better to just cleanly add it in examples.