chore: add fourmolu pre-commit hook

Installs a pre-commit hook that checks all staged .hs files are
formatted by fourmolu before allowing a commit. Configured via
core.hooksPath = .githooks.

Run: ./hs fourmolu -i <files>  to format

.githooks/pre-commit

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Pre-commit hook: format Haskell sources with fourmolu before each commit.
+# Aborts the commit if formatting changes are needed.
+#
+# To install: make sure .githooks is CONFIGURED as the hooks directory
+# in git config (or copy this to .git/hooks/pre-commit).
+set -euo pipefail
+
+# Only run if fourmolu is available (skips on systems without the hs wrapper)
+if command -v ./hs &>/dev/null && ./hs fourmolu --version &>/dev/null 2>&1; then
+    # Check which .hs files are staged for commit
+    STAGED_FILES=$(git diff --cached --name-only --diff-filter=ACM | grep '\.hs$' || true)
+    if [ -n "$STAGED_FILES" ]; then
+        # shellcheck disable=SC2086
+        if ! ./hs fourmolu -m check $STAGED_FILES >/dev/null 2>&1; then
+            echo "❌ Formatting check failed. Run the following to fix:"
+            echo "   ./hs fourmolu -i $STAGED_FILES"
+            echo "   git add $STAGED_FILES"
+            exit 1
+        fi
+    fi
+fi