fix(WP-0014/A2): fix devenv shell — IHP_LIB, ghci, ghcid now available

Root cause: devenv shell reads devenv.nix only; IHP's Haskell setup is in
flake.nix's devenv.shells.default which is only loaded by nix develop.

Fix: create devenv.yaml to register IHP as a devenv input (pinned to the
same rev as flake.lock) with overlays.default applied.  Update devenv.nix
to set IHP_LIB/IHP env vars and enable languages.haskell with pkgs.ghc
(IHP-overlay GHC) + all project packages; add ghcid explicitly.

devenv.lock pins IHP at df3922d (matches flake.lock).

scripts/compile-check: updated header comments and added IHP_LIB echo;
switches from ghcid to ghci one-shot (ghcid not available in all shells).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

devenv.nix

@@ -8,17 +8,61 @@
 # 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, ... }:
+# devenv.nix — devenv v2 shell configuration.
+#
+# `devenv shell` reads only this file (not flake.nix's devenv.shells.default).
+# IHP's flake module configures devenv.shells.default — that is only visible
+# to `nix develop`.  This file therefore duplicates the IHP-critical parts so
+# that `devenv shell` also has a working Haskell + IHP_LIB environment.
+#
+# devenv.yaml registers IHP as a devenv input (pinned rev matching flake.lock)
+# and applies IHP's nixpkgs overlay, making pkgs.ghc the IHP Haskell package
+# set (ihp, ihp-hspec, ghcid, etc.).
 {
-  # 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.
+  # ── Resource limits (constrained host: 2 CPU / ~3.8 GiB RAM) ─────────────
+  # -A32m: smaller minor heap; -M2g: hard heap ceiling.
+  # -N1 omitted: requires -threaded, which breaks IHP tools.
+  # GHC parallel module compilation is capped via -j1 in .ghci.
   env.GHCRTS = "-A32m -M2g";
 
-  # Make tailwindcss available in process scripts (devenv v2 builds process
-  # scripts with only packages declared here, not from flake ihp.packages).
-  packages = [ pkgs.nodePackages.tailwindcss ];
+  # ── IHP environment variables ─────────────────────────────────────────────
+  # IHP_LIB is read by .ghci (applicationGhciConfig) and by scripts/compile-check.
+  # IHP is the legacy name used by older Makefiles.
+  env.IHP_LIB = "${inputs.ihp.packages.${pkgs.stdenv.system}.ihp-env-var-backwards-compat}";
+  env.IHP     = "${inputs.ihp.packages.${pkgs.stdenv.system}.ihp-env-var-backwards-compat}";
 
+  # ── Haskell toolchain ─────────────────────────────────────────────────────
+  # IHP's nixpkgs overlay (applied via devenv.yaml overlays) makes pkgs.ghc
+  # the IHP-augmented Haskell package set.  pkgs.ghc.ghc is the GHC compiler.
+  languages.haskell.enable = true;
+  languages.haskell.package = pkgs.ghc.ghc.withPackages (p: with p; [
+    p.ihp
+    # Project runtime dependencies (mirrors flake.nix haskellPackages)
+    http-conduit
+    aeson
+    string-conversions
+    cryptohash-sha256
+    base16-bytestring
+    random-bytestring
+    yaml
+    network-uri
+    # Dev / test tools
+    cabal-install
+    hlint
+    hspec
+    p.ihp-hspec
+  ]);
+  # ghcid is used by scripts/compile-check-core; not auto-added by devenv.
+  # LSP disabled — heavy binary not needed for CI-style compile checks.
+  languages.haskell.lsp.enable = false;
+
+  # ── Extra packages ────────────────────────────────────────────────────────
+  packages = [
+    pkgs.ghc.ghcid
+    pkgs.nodePackages.tailwindcss
+  ];
+
+  # ── Processes ─────────────────────────────────────────────────────────────
   # 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";
 }