how do i delete photos from my old MacBook Air w/o deleting them from iCloud?

Do you want to delete only all of the photos on your Mac or only some of them?

If you want to delete all of them, that's easy. Just delete the library itself (or erase the Mac entirely if you are retiring it). The photos and videos in iCloud will be unaffected.

If you only want to delete some of them but can live without any future synchronization with the cloud or other devices, that's also easy. Just disable iCloud Photos on the Mac before doing the deletions there. Just know that if you ever enable it again, those photos will go up to the cloud then and any new photos in the cloud from other devices will come down to the Mac.

What you can't do is keep the Mac connected to iCloud Photos and have different contents in the two. The primary point of iCloud Photos is to keep the content on all of the devices (iPhones, Macs, iPads) connected to the same cloud library the same.