docs: Add AGENTS.md to allow LLMs to work effectively with the codebase

[johnsaigle]

LLM agent tools are very popular for developers and auditors alike. Providing an AGENTS.md file should allow them to be more effective with their tools. We may as well add one here because people using these tools will be creating their own versions of it anyway.

I used the nomenclature from https://agents.md because it seems to be the most general/least vendor-specific, but I'm open to using a different style or name.

It's important to keep these files pretty concise in order to not waste context window space or otherwise 'poison' the window with irrelevant or misleading info. As a result, I tried to avoid content that's already well-covered by README, CONTRIBUTING, and DEVELOP or items that are already handled by CI.

[djb15]

I don't have a good idea what a strong AGENTS.md looks like, but I don't see any reason why we shouldn't add this and try it out. We can always modify it over time

[johnsaigle]

@djb15 For reference, most of these tools have some sort of init command that scans existing tests, makefiles, build scripts and so on to auto-generate the AGENTS.md (or equivalent). This one was generated by opencode.

We can absolutely iterate on the exact contents and try multiple approaches.