tsk-45: drop stale README xattr/symlink prereqs
The git-ref backend stores tasks in refs/tsk/* inside the host repo,
not in a .tsk/ working-tree directory, so it neither needs xattrs nor
filesystem symlink support. Replace those prereqs with a one-line note
that pointing at git's own filesystem requirements is enough.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>