feat: added AGENTS.md file icon

[milkyware]

Description

Bit of a follow on from #3336. Added a file icon for agents.md. Opted to reuse the motive icon from the folder-robot icon for continuity rather than the existing robot or robots file icons

Contribution Guidelines

• By creating this pull request, I acknowledge that I have read the Contribution Guidelines for this project
• I have read the Code Of Conduct and promise to abide by these rules

[github-actions]

Preview

Thank you for creating a pull request. This preview shows you how your icon will look on the different themes:

Check how your icon fits in a 16x16 grid with our Pixel Perfect Checker by following this link.

You can find more information on how to contribute in the contribution guidelines.

[SayanShankhari]

to snap into grid, can you please try this one
[remove the grid element before pushing]

[milkyware]

I've opted to rescale the icon to better snap to the grid and retain the rounded corners as I think that offers more continuity to the folder icon @SayanShankhari

[SayanShankhari]

actually it's to avoid the blurry pixels at 16x16 default resolution, keeping non square shape when it's not absolutely required makes it blurry,
i have no problem if maintainers approve