nix/docs: drop dead walk helper + dedupe checks.docs eval

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.

flake.nix

@@ -347,16 +347,10 @@
           # Nix options docs evaluation (#616). Cheap: pulls in
           # `nixosOptionsDoc` + the host module's stub eval, no rust or
           # frontend deps. CI fails fast if a module change breaks
-          # option declarations or the doc rendering. `docs-host` +
-          # `docs-agent` are exposed as `packages` outputs (not checks)
-          # — they share this eval, so building the bundle here covers
-          # both surfaces in one shot.
-          docs =
-            (import ./nix/docs.nix {
-              inherit pkgs self;
-              inherit (nixpkgs) lib;
-              inherit (nixpkgs.lib) nixosSystem;
-            }).bundle;
+          # option declarations or the doc rendering. Reuses the
+          # `packages.<system>.docs` derivation so the per-system eval
+          # of `nix/docs.nix` happens once.
+          inherit (self.packages.${system}) docs;
         }
       );
     };