devenv v2 requires devenv.nix to exist even when the full shell
configuration lives in flake.nix via IHP's devenv.flakeModule.
Contains project-specific overrides (GHCRTS, tailwind process);
IHP core (postgres, ghcid, RunDevServer) still comes from the flake module.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
