argus review notes on #618:
- `walk` in `pickSubtrees` was leftover from an earlier traversal
design; `pick` does everything we need. drop it.
- `checks.docs` was re-importing `nix/docs.nix` independently of
`packages.docs`; the comment claimed they shared eval but they
didn't (nix's lazy eval + import caching made the *result*
identical, not the eval). switch to `inherit (self.packages.\${system}) docs;`
so the check is literally the package output, no second import.
24f61f3386