fix(pi-file-search): harden fd/rg against observed live failure modes

- rg: exit 2 with stdout matches (broken node_modules symlinks) now
  returns matches with a 'some paths were unreadable' note instead of
  throwing; --no-messages was verified to keep exit 2 and was rejected
  to preserve real error reporting
- rg: patterns containing a literal newline or \n escape auto-enable
  --multiline
- fd: patterns starting with * or ? are auto-treated as globs (with a
  note) instead of failing regex parse
- fd: dotfile patterns retry once with hidden files included
- fd: non-directory search paths fail fast with the resolved path and
  cwd in the error

Adds 11 vitest cases (tmp-dir fixtures incl. broken symlink and
unreadable dir), updates README and the file-search skill.