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.yaml

@@ -0,0 +1,7 @@
+inputs:
+  ihp:
+    # Pinned to the same rev as flake.lock so devenv shell and nix develop
+    # use identical IHP packages.  Update by running: devenv update
+    url: github:digitallyinduced/ihp/df3922d1a7166b131674efa3d3555ed7195ddf70
+    overlays:
+      - default