Keep Markdown paste within what the room accepts

Run the server's introduced-URL checks, the byte limit, and the document's
image and depth limits before converting; keep numbered lists that do not
start at one as lines; end lists at unindented prose; make detection
linear and skip it above the source limit; recognise Shift+Mod+V on any
layout and with Option.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>