mr: a merge request can say it is not asking yet !240

Stacked on !239.

mr create --draft, mr draft, mr ready, plus the web toggle and a chip on both MR pages. A draft does not merge and does not appear in anyone's review queue.

Draft is a column, not a fifth state. State is open | merged | closed | source_gone, and adding to it would change what every state = 'open' query means — the merge gates, the review queue, the list filters. A draft is open; it is just not asking.

The merge refusal is unconditional, unlike every other gate. The others are settings an admin turns on; this one is the author's own statement about their own work, so there is nothing to configure.

This is the first of the three things #111 asks for, in the order the issue asks for them ("a draft flag first; a pending-review batch second"). Batched review and range-diff remain open. The fourth bullet it lists — mr diff truncating silently — already landed in v1.11.0 with #117, so the issue is one item staler than it reads.

Ref #111