Function enable 

pub fn enable(root: &Path) -> bool

Enable Impact tracking. Returns whether it was newly enabled (false if already on).

Also ensures .fallow/ is gitignored so the store is not accidentally committed: the store is the feature’s local-only promise, and enable is the moment it is first created, so it is the right place to make “gitignored, never uploaded” true even when the user never ran fallow init. Best-effort: a gitignore write failure must never fail enabling.