Add plugin directory assets

[shaunandrews]

Summary

• Add icon and banner assets for the WordPress.org plugin directory
• Includes 1x and 2x versions of both icon (128×128, 256×256) and banner (772×250, 1544×500)

🤖 Generated with Claude Code

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Co-authored-by: shaunandrews <shaunandrews@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[jeffpaul]

@shaunandrews any chance you can add an icon.svg variant as well?

[jeffpaul]

Looks great, thanks!