meta: add a root-level AGENTS.md

[jasnell]

We're seeing a number of repeated mistakes in AI-generated contributions, including agents using Signed-off-by, creating what appear to be unsupervised PRs, appearing to respond to comments without human oversight, etc. Having an AGENTS.md file with specific instructions to the agent on what to avoid should help.

[legendecas]

There is an ealier PR for AGENTS.md: #65463

[joyeecheung]

The document uses a tone as if it's speaking to a human, even though it is not supposed to be read by human? e.g. the distinction of collaborators or non-collaborators is pointless to an agent unless you give it permisssions where the distinction makes a difference, but then the document goes on saying the agent shouldn't use any of these permissions, which means the agent is not supposed to make use of that distinction in the first place.

[jasnell]

The document uses a tone as if it's speaking to a human, even though it is not supposed to be read by human? e.g. the distinction of collaborators or non-collaborators is pointless to an agent unless you give it permisssions where the distinction makes a difference, but then the document goes on saying the agent shouldn't use any of these permissions, which means the agent is not supposed to make use of that distinction in the first place.

Agents are perfectly capable of correctly interpretting it as written.

[jasnell]

Applied formatting.

[jasnell]

@mcollina are you satisfied with the updates enough to clear your "request changes"?

[BethGriggs]

It might be worth adding some instructions relating to scanning for vulnerabilities and drafting security reports too, but that could be a follow-up so not to derail this landing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.16%. Comparing base (46a7dbd) to head (30617f6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65493      +/-   ##
==========================================
- Coverage   90.21%   90.16%   -0.05%     
==========================================
  Files         751      751              
  Lines      253550   253550              
  Branches    47813    47764      -49     
==========================================
- Hits       228733   228615     -118     
- Misses      16076    16194     +118     
  Partials     8741     8741              

see 31 files with indirect coverage changes

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