nixos: replace rcm with home-manager dotfile links

nixos/dotfiles-links.nix

@@ -0,0 +1,65 @@
+{ config, lib, ... }:
+let
+  # Replicate the useful part of rcm/rcup:
+  # - dotfiles live in ~/dotfiles/dotfiles (no leading dots in the repo)
+  # - links in $HOME add a leading '.' to the first path component
+  # - link files individually so unmanaged state can coexist (e.g. ~/.cabal/store)
+  #
+  # Use out-of-store symlinks so editing the repo updates config immediately
+  # without a rebuild/switch (only link *placement* is managed by HM).
+  oos = config.lib.file.mkOutOfStoreSymlink;
+
+  # Where the checked-out repo lives at runtime (activation time).
+  worktreeDotfiles = "${config.home.homeDirectory}/dotfiles/dotfiles";
+
+  # Use the flake source for enumeration (pure), but point links at the worktree.
+  srcDotfiles = ../dotfiles;
+
+  excludedTop = [
+    # Managed by Nix directly (PATH/fpath), not meant to appear as ~/.lib.
+    "lib"
+    # Avoid colliding with HM-generated xdg.configFile entries for now.
+    "config"
+    # Handled as a single directory symlink below.
+    "emacs.d"
+  ];
+
+  firstComponent = rel:
+    let parts = lib.splitString "/" rel;
+    in lib.elemAt parts 0;
+
+  isExcluded = rel: lib.elem (firstComponent rel) excludedTop;
+
+  listFilesRec = dir:
+    let
+      entries = builtins.readDir dir;
+      names = builtins.attrNames entries;
+      go = name:
+        let
+          ty = entries.${name};
+          path = dir + "/${name}";
+        in
+          if ty == "directory" then
+            map (p: "${name}/${p}") (listFilesRec path)
+          else
+            [ name ];
+    in
+      lib.concatLists (map go names);
+
+  managedRelFiles =
+    lib.filter (rel: !(isExcluded rel)) (listFilesRec srcDotfiles);
+
+  mkManaged = rel:
+    lib.nameValuePair ".${rel}" {
+      source = oos "${worktreeDotfiles}/${rel}";
+    };
+in
+{
+  home.file =
+    (builtins.listToAttrs (map mkManaged managedRelFiles))
+    // {
+      # Keep ~/.emacs.d as a directory symlink (matches current setup).
+      ".emacs.d".source = oos "${worktreeDotfiles}/emacs.d";
+    };
+}
+