fix: pin Nix lookup outside login shells

Set the global nix-path from the declarative NIX_PATH list so a clean environment resolves the same locked Nixpkgs. Keep old root channel profiles for recovery rather than deleting them to silence the activation warning.

README.md

@@ -55,7 +55,7 @@ nix --extra-experimental-features 'nix-command flakes' build \
 
 Then review/apply the resulting closure as above. The configuration creates `dev`, sets permissions and deploys its files. **No separate Neovim clone, copy, useradd, chown or global Git-config recipe is required.**
 
-This host build does not use a mutable channel. NixOS's native flake integration also pins the `nixpkgs` registry entry and `<nixpkgs>` lookup to the system input. Dev-environment templates/composition remain deferred; there is no flake framework here.
+This host build does not use a mutable channel. NixOS's native flake integration pins the `nixpkgs` registry entry and login-shell `<nixpkgs>` lookup to the system input; the global `nix-path` setting keeps the same pin when `NIX_PATH` is unset. Old root channel profiles are retained for recovery, not used as build inputs. Dev-environment templates/composition remain deferred; there is no flake framework here.
 
 ## Neovim: import, do not rewrite
 

configuration.nix

@@ -1,4 +1,9 @@
-{ inputs, modulesPath, ... }:
+{
+  config,
+  inputs,
+  modulesPath,
+  ...
+}:
 
 {
   imports = [
@@ -13,6 +18,8 @@
 
   nix = {
     channel.enable = false;
+    # Keep non-login environments on the same pin when NIX_PATH is unset.
+    settings.nix-path = config.nix.nixPath;
     settings.experimental-features = [
       "nix-command"
       "flakes"