fix(scripts): stop the pre-commit hook from blocking commits on older revisions

The installed hook exec'd scripts/preflight.sh unconditionally. That hook is a
local, untracked artifact, so it survives a checkout of any commit that predates
the script, and on such a commit exec failed with a bare "No such file or
directory" — blocking the commit with nothing to act on.

In this repository scripts/preflight.sh first appears at 8e12401, so every earlier
commit is affected. The failure is also silent in a dangerous way: it swallowed
a `git commit --amend`, so a commit-message rewrite reported as applied when it
had not been, and had to be redone.

The hook now checks the script is executable and exits 0 otherwise, so a revision
without the pre-flight simply skips it. Normal operation is unchanged.

Also honours core.hooksPath. git ignores .git/hooks entirely once that is set, so
the hook was being written to a location that would never run while this script
still reported success. No repository sets it today, so that part is latent.

Author: vistart

scripts/install-git-hooks.sh

@@ -4,21 +4,42 @@
 # Installs an optional pre-commit hook that runs scripts/preflight.sh so a
 # broken import/lint is caught before the commit (and before a CI round-trip).
 # Bypass a single commit with: git commit --no-verify
+#
+# Re-run this after pulling, and re-run it from the repository root. To remove
+# the hook, delete .git/hooks/pre-commit (or the core.hooksPath directory).
 set -euo pipefail
 
 cd "$(dirname "$0")/.."
 
-HOOK=".git/hooks/pre-commit"
-if [ ! -d .git/hooks ]; then
-  echo "install-git-hooks: .git/hooks not found (not a git repo?)" >&2
-  exit 2
+# git ignores .git/hooks entirely once core.hooksPath is set, so a hook written
+# there would never run while this script still reported success.
+HOOKS_PATH="$(git config core.hooksPath || true)"
+if [ -n "$HOOKS_PATH" ]; then
+  echo "install-git-hooks: core.hooksPath is set to '$HOOKS_PATH'." >&2
+  echo "install-git-hooks: .git/hooks is ignored by git; writing the hook to" >&2
+  echo "                 '$HOOKS_PATH/pre-commit' instead." >&2
+  HOOK="$HOOKS_PATH/pre-commit"
+  mkdir -p "$HOOKS_PATH"
+else
+  HOOK=".git/hooks/pre-commit"
+  if [ ! -d .git/hooks ]; then
+    echo "install-git-hooks: .git/hooks not found (not a git repo?)" >&2
+    exit 2
+  fi
 fi
 
 cat > "$HOOK" <<'HOOK_BODY'
 #!/usr/bin/env bash
 # Installed by scripts/install-git-hooks.sh — runs the local pre-flight.
 # Skip with: git commit --no-verify
-exec "$(git rev-parse --show-toplevel)/scripts/preflight.sh"
+#
+# The guard below matters on older revisions: this hook is a local, untracked
+# artifact, so it survives a checkout of a commit that predates
+# scripts/preflight.sh. Without the guard, exec fails with a bare
+# "No such file or directory" and blocks the commit with nothing to act on.
+PREFLIGHT="$(git rev-parse --show-toplevel)/scripts/preflight.sh"
+[ -x "$PREFLIGHT" ] || exit 0
+exec "$PREFLIGHT"
 HOOK_BODY
 chmod +x "$HOOK"