Adds a "Recently Evergreened" section to /garden/ showing posts that
reached evergreen status within the last 90 days, with a dedicated
green celebration card style.
- New `recentEvergreens` Eleventy collection (evergreeSince within 90d)
- garden.njk: conditional celebration section above the stage groups
- tailwind.css: .garden-evergreen-celebration card (evergreen palette)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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