plain() deleted those characters everywhere, so "join #lotus-general"
lost its #, snake_case and @some_user lost underscores, "->" became
"-", "5 * 3" became "5 3", and "<b>" became "<b". Strip only syntax:
heading / quote / list markers at line start, paired **x** *x* _x_ `x`,
and HTML tags.
Fixes#308
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5