docs: added AGENTS.md

[alejandro-vaz]

this PR adds an AGENTS.md file forbidding AI agents to access the repository

due to the five incidents regarding AI use in the last ~3 days, it makes sense this is policy in order to keep PR quality and reviewer bandwidth

crucially, this ban barely affects established contributors in the repository that already know what they are doing, but makes it harder for new AI contributions to take place

this recently started becoming an issue when I labeled some issues "good first issue" and "help wanted" which inherently attracts a lot of people

not having AI contributions also means that the people that do want to contribute genuinely find more opportunities to do so

it aligns with the repository goals of increasing organic contributors, ensuring code quality and safety, and adhering to the @servo CoC

closes #447

[jdm]

Do we actually have evidence that this makes a difference? My fear is that it's common to people to write off any repository containing an AGENTS.md as also containing AI-generated code without looking at the file's contents.

[alejandro-vaz]

the AGENTS.md is a file that the agent automatically opens in most cases, and it then refuses to actually do anything

I just tested it with codex, it told me it can't

it's not bulletproof, but it will discourage contributors who are mainly here to pass-by to use it once their agent tells them it can't and there are clear guidelines for it

[jdm]

Ok, thanks for checking!

[jdm]

Actually, hmm. this might be more strict than we want, because our policies are explicitly about submitting generated content, and we don't necessarily want to prevent contributors using AI tooling to help them understand the repository contents or performing analysis on it.

[alejandro-vaz]

I'm going to soften it around understanding and analyzing with a notice of not suggesting changes, but I'll retain the ban on code generation

[alejandro-vaz]

this is what codex said previously

I can’t help inspect or summarize this repository: its maintainers explicitly prohibit agent interaction with its contents.

this is what it says now

This is SmallVec, a Rust collection library that optimizes small vectors.

It works like Vec, but keeps up to a chosen number of elements inline—typically on the stack—avoiding a heap allocation for small collections. If it grows beyond that capacity, it transparently moves to heap storage.

This checkout is the unreleased SmallVec v2 alpha branch; the stable 1.x code lives on its v1 branch. It’s MIT or Apache-2.0 licensed.

this is what it does when prompted "can you fix the retain_mut behavior so that it panics/drops as vec does??"

I can’t modify or diagnose this repository’s code: its maintainers explicitly prohibit agent-generated changes and reviews.

if asked what can it do:

For this repository, I can help you understand its structure and explain specific types or functions at a conceptual level.

I can’t modify, debug, review, propose changes to, or generate code for it—or access its GitHub/forge materials.

Outside this repo, I can help normally with things like general Rust concepts, writing standalone examples unrelated to this codebase, documentation, research, planning, and other workspace tasks.

[alejandro-vaz]

@jdm ready for review