chore: Add basic AGENTS.md

[slorber]

Motivation

Introduces a very basic AGENTS.md file for AI coding agents.
This file is always loaded into context, so let's keep it minimal for now, test the current setup, and eventually add more specialized skills later.

This is not a way for us to encourage our community to send AI-generated PRs.

Quite the opposite: we'd like to force AI users to disclose their AI usage, and avoid or have a way to detect slop PRs easily (see the last section). Inspired by this line:
https://github.com/ghostty-org/ghostty/blob/6692be477d40f9eba49b861aa63ac67640b4bb4e/AGENTS.md?plain=1#L31-L32

Test Plan

?

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⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🔴 41	🟢 98	🟢 100	🟢 100	Report
/docs/installation	🟠 58	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 71	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 66	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 66	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 67	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 71	🟢 100	🟢 100	🟠 86	Report