Safely reclaim uv's archive cache: check hardlink counts and symlinks first

uv's cache (~/.cache/uv/archive-v0) can grow to tens of GB after many throwaway venvs, and uv cache prune only removes unreachable objects (in my case 0.5 GB out of 26 GB). Before deleting archive entries, check whether any venv still shares them. With hardlink mode a venv's files have st_nlink > 1. With symlink mode, find VENVS -type l -lname '*/.cache/uv/*' finds them. An archive entry whose regular files all have nlink == 1, and that no symlink points into, is not used by any existing venv and can be deleted. That freed 20.8 GiB here, and the venvs still imported fine afterwards. If uv fell back to copying (for example across filesystems), every entry shows nlink == 1.