chore: harden pre-commit hook (tracked .githooks/ + core.hooksPath)

Replace the dev-shell installer that wrote .git/hooks/pre-commit with a
tracked .githooks/pre-commit wired via `git config core.hooksPath .githooks`.

Fixes three issues with the old hook: it gated on `[ -d .git ]` (false in
worktrees/submodules where .git is a file), it clobbered any existing
.git/hooks/pre-commit on every `nix develop`, and `nix fmt … || exit 0`
swallowed real formatter failures. The new hook skips only when `nix` is
absent (CI is the authoritative gate) and otherwise blocks on a `nix fmt`
failure or reformat.

Author: Unisay

.githooks/pre-commit

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+# Content-based format gate. CI's `nix fmt && git diff --exit-code` is
+# authoritative; this hook is just a local convenience.
+#
+# Tracked under .githooks/ and wired via `git config core.hooksPath .githooks`
+# in the dev-shell shellHook — robust across git worktrees and submodules
+# (where .git is a file) and it never clobbers a developer's .git/hooks.
+set -u
+command -v nix >/dev/null 2>&1 || exit 0 # no nix here → skip; CI still gates
+
+before=$(git diff)
+if ! nix fmt >/dev/null 2>&1; then
+  echo "pre-commit: 'nix fmt' failed — fix the formatter error, then commit." >&2
+  exit 1
+fi
+if [ "$before" != "$(git diff)" ]; then
+  echo "pre-commit: 'nix fmt' reformatted files — re-stage them, then commit." >&2
+  exit 1
+fi

.gitignore

@@ -3,4 +3,5 @@
 !/.github/
 !/.tidyrc.json
 !/.lua-format
+!/.githooks/
 /output/

flake.nix

@@ -48,23 +48,9 @@
             spago-bin.spago-0_21_0
             treefmt
           ];
-          # Install a content-based pre-commit hook. It compares the working
-          # tree diff before and after `nix fmt`, so it only objects to changes
-          # the formatter itself introduces (not the developer's existing
-          # unstaged work) and is not fooled by formatters that only bump mtime.
-          # Rewritten each shell entry to stay in sync with this flake.
-          shellHook = ''
-            hook=.git/hooks/pre-commit
-            if [ -d .git ]; then
-              printf '%s\n' \
-                '#!/usr/bin/env bash' \
-                'before=$(git diff)' \
-                'nix fmt >/dev/null 2>&1 || exit 0' \
-                '[ "$before" = "$(git diff)" ] || { echo "nix fmt changed files; re-stage them, then commit." >&2; exit 1; }' \
-                > "$hook"
-              chmod +x "$hook"
-            fi
-          '';
+          # Robust pre-commit hook: point git at the tracked .githooks/ dir
+          # (worktree/submodule-safe; never clobbers an existing .git/hooks).
+          shellHook = "git config core.hooksPath .githooks";
         };
       }
     );