Introduce agents.md

[PromoFaux]

What does this PR aim to accomplish?:

Adds an AGENTS.md file following the agents.md standard: a predictable place for AI coding agents to find the repository layout, build/test commands and contribution requirements (DCO sign-off, branch targeting, code style) without them having to rediscover it every session. It is just standard markdown, and no tooling or workflow changes come with it.

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[rdwebdesign]

Tests are failing.

Maybe we need to add \"!**/.git/**\" to this line:

docs/package.json

Line 20 in f2f2955

"markdownlint": "markdownlint-cli2 \"**/*.md\" \"!**/node_modules/**\"",

Like this:

    "markdownlint": "markdownlint-cli2 \"**/*.md\" \"!**/node_modules/**\" \"!**/.git/**\"",

[yubiuser]

Or we remove the hard tabs it complains about.

[rdwebdesign]

Or we remove the hard tabs it complains about.

The tests are complaining about hard tabs in .git hidden directory.
These files are created by git command when the repo is cloned. They don't exist in our code.