replace the boolean fence toggle in _mask_prose_link_destinations with
proper (marker_char, opener_length) tracking so a mismatched-markdown-fence
body or an indented code block cannot re-enable prose-masking over live
command lines. closes an exploitable bypass in the community-source
install path; plugin_guard inherits the fix through scan_file.
also tighten is_indented_code to treat any tab indent (single or double)
as code, per CommonMark §4.4.
JulianCruzet
725713d575