feat(nixos): migrate system configuration

configs/nixos/modules/shell/direnv/default.nix

@@ -0,0 +1,26 @@
+{
+  config,
+  lib,
+  ...
+}:
+# direnv for the primary user, configured through home-manager.
+let
+  cfg = config.modules.shell.direnv;
+  user = config.user.name;
+in
+{
+  options.modules.shell.direnv.enable =
+    lib.mkEnableOption "direnv for the primary user, with nix-direnv shell caching";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.programs.direnv = {
+      enable = true;
+      silent = true;
+
+      # nix-direnv caches the evaluated shell so re-entering a directory is instant
+      # instead of re-running `nix develop`, and adds the `use flake` stdlib helper.
+      nix-direnv.enable = true;
+      stdlib = builtins.readFile ./repo-flake.sh;
+    };
+  };
+}

configs/nixos/modules/shell/direnv/repo-flake.sh

@@ -0,0 +1,33 @@
+if declare -f use_flake >/dev/null; then
+  __dotfiles_use_flake_definition="$(declare -f use_flake)"
+  eval "${__dotfiles_use_flake_definition/use_flake/__dotfiles_nix_direnv_use_flake}"
+  unset __dotfiles_use_flake_definition
+
+  use_flake() {
+    local flake_expr="${1:-.}"
+
+    if [[ "$flake_expr" == //* ]]; then
+      local git_root
+      if ! git_root="$(git rev-parse --show-toplevel 2>/dev/null)"; then
+        log_error 'use flake // requires a Git repository'
+        return 1
+      fi
+
+      local flake_suffix="${flake_expr#//}"
+      if [[ -n "$flake_suffix" && "$flake_suffix" != \#* && "$flake_suffix" != /* ]]; then
+        flake_expr="path:${git_root}/${flake_suffix}"
+      else
+        flake_expr="path:${git_root}${flake_suffix}"
+      fi
+
+      shift
+      __dotfiles_nix_direnv_use_flake "$flake_expr" "$@"
+      return
+    fi
+
+    __dotfiles_nix_direnv_use_flake "$@"
+  }
+else
+  log_error 'nix-direnv must define use_flake before the // wrapper loads'
+  return 1
+fi