The inline JS in the Write PreToolUse hook had a multi-layer escaping
bug: the regex [\\/\\] collapsed to [\/\] after the validator's
unescape chain, producing an invalid regex (Unmatched ')').
Fix: move the doc-file-warning hook to scripts/hooks/pre-write-doc-warn.js,
eliminating the inline escaping problem entirely. All 992 tests now pass.
Closes the 991/992 CI failure on main.