Add fourmolu + hlint with scripts and git hooks (Atlas-style)

- Create scripts/build, scripts/test, scripts/run, scripts/install-hooks
- Create git-hooks/pre-commit (auto-format staged .hs files)
- Create git-hooks/pre-push (check formatting + hlint before push)
- Handle git worktrees correctly in hook installation
- Run fourmolu on all source files to fix existing formatting

scripts/git-hooks/pre-commit

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Pre-commit hook: format staged Haskell files with fourmolu.
+# Installed by: scripts/install-hooks
+set -euo pipefail
+
+PROJECT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/../.." && pwd)"
+
+# Get staged .hs files (handles filenames with spaces)
+staged=$(git -C "$PROJECT_DIR" diff --cached --name-only --diff-filter=ACM -- '*.hs')
+if [ -z "$staged" ]; then
+    exit 0
+fi
+
+# Collect the files into an array
+mapfile -t files <<< "$staged"
+
+echo "--- Formatting staged Haskell files with fourmolu..."
+if ! "$PROJECT_DIR/hs" fourmolu --mode inplace "${files[@]}"; then
+    echo "ERROR: fourmolu formatting failed" >&2
+    exit 1
+fi
+
+# Re-stage any files that were modified by formatting
+changed=0
+for f in "${files[@]}"; do
+    if ! git -C "$PROJECT_DIR" diff --quiet "$f"; then
+        git -C "$PROJECT_DIR" add "$f"
+        changed=$((changed + 1))
+    fi
+done
+
+if [ "$changed" -gt 0 ]; then
+    echo "--- Formatted $changed file(s) — re-staged automatically"
+fi