Review model: no draft MRs, no batched review, no range-diff #111

• mr create has no --draft (mr.go:35).
• A review is one verdict per call (mr.go:783); diff comments post immediately (diffcomment.go:33). A reviewer cannot compose a review and submit it as a unit.
• Any head change stales every review (store/mrs.go:239) and threads by SHA equality (diffcomment.go:175), with nothing showing what changed between pushes.
• mr diff truncates silently at 4 MiB (gitutil/merge.go:103, mr.go:635).

Remedy: a draft flag first; a pending-review batch second; a truncation notice today.