chore(features): drop orphan userPackages reader

`features/default.nix` had a let-block that read
`~/.config/home-manager/user-packages.json` at eval time via
`builtins.pathExists` + `builtins.readFile`, parsed it as JSON, and
filtered to valid pkgs — then never appended the result to
`home.packages` or anywhere else. The `userPackages` variable was
completely orphan.

Two problems with the dead code: (1) it was an undocumented hidden
mechanism (no docs mentioned `user-packages.json`), (2) it made flake
evaluation impurely depend on a user's home directory for no payoff —
flake outputs would silently differ between machines depending on the
presence of that file, even though nothing in the build used it.

Removed the let-block entirely. The nomarchyLib import stays.

Found during Pillar 8 audit of features/apps.

features/default.nix

@@ -2,14 +2,6 @@
 
 let
   nomarchyLib = import ../lib { inherit lib; };
-  userPackagesFile = "${config.home.homeDirectory}/.config/home-manager/user-packages.json";
-  userPackages = if builtins.pathExists userPackagesFile then
-    let
-      pkgNames = builtins.fromJSON (builtins.readFile userPackagesFile);
-      # Filter to only packages that exist in pkgs to prevent build failures
-      validPkgs = builtins.filter (name: builtins.hasAttr name pkgs) pkgNames;
-    in builtins.map (name: pkgs.${name}) validPkgs
-  else [];
 in
 {
   imports = [