fix(nix): out-of-store symlink for nvim so Lazy can write lazy-lock
xdg.configFile."nvim".source copied the config into the read-only Nix
store, so LazyVim's :Lazy sync/update failed with "Permission denied"
writing lazy-lock.json. Use mkOutOfStoreSymlink to point ~/.config/nvim
at the live repo instead; also makes .lua edits live without a rebuild.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>