fix(WP-0014/A1): add devenv.nix required by devenv v2+

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>

devenv.nix

@@ -0,0 +1,20 @@
+# devenv.nix — required by devenv v2+.
+#
+# IHP's flake module (flake.nix) provides the full devenv shell via
+# `inputs.devenv.flakeModule`: Haskell toolchain, PostgreSQL, ghcid,
+# and RunDevServer. This file satisfies devenv v2's file-presence
+# requirement and adds project-specific overrides on top.
+#
+# Duplicates the settings from flake.nix devenv.shells.default so they
+# are applied under both devenv v1 (flake path) and devenv v2 (this file).
+{ pkgs, lib, config, inputs, ... }:
+{
+  # Resource limits for constrained host (2 CPU / 3.8 GiB RAM).
+  # -A32m: smaller minor heap, -M2g: hard heap ceiling.
+  # Note: -N1 omitted intentionally (requires -threaded, breaks IHP tools).
+  # GHC parallel module compilation is capped via -j1 in .ghci instead.
+  env.GHCRTS = "-A32m -M2g";
+
+  # Tailwind CSS watcher — not part of IHP's core devenv module.
+  processes.tailwind.exec = "tailwindcss -c tailwind/tailwind.config.js -i ./tailwind/app.css -o static/app.css --watch=always";
+}