feat: add icons for AGENTS.md file

[marlondecol]

Description

This pull request introduces a new icon for the AGENTS.md file, including a variant for light themes. Closes #3310.

This is yet another suggestion, since pull requests #3241 and #3437 also propose icons for the same file. However, the icon in this pull request is based on the favicon of agents.md, which appears to be the official website for the format.

Design details

The icon was manually adapted to improve sharpness at small sizes, while staying as faithful as possible to the original.

Original agents.md favicon

The icon is intentionally not perfectly snapped to the 16 px grid. 2 px strokes felt too thin, but 3 px strokes felt too thick; the stroke weight and vertex positions in this version are, to my eye, the adjustments that yielded the most balanced result I could achieve.

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As always, suggestions and improvements are welcome!

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[github-actions]

Preview

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[lucas-labs]

Hi there!
Maybe we could unify the motive and use the same icon for this one and agents folders (#3446)?

I know this one comes from the AGENTS.md "specification". But it's kind of the same concept. Maybe I'm wrong.

[marlondecol]

Hey @lucas-labs!

Since AGENTS.md refers only to this file in particular, in my opinion it would be better not to generalize the icon to agent files and folders of other formats. Also, this format is not related to creating custom agents per se, but rather to specifying instructions that are provided to agents, so the concepts are not quite the same.