Comment on the plan from the sidecar

One list rather than two tabs: an accepted comment is a decision in the same
sense a questionnaire's answer is, and would otherwise have to pick a side.
Outstanding items first in document order, resolved behind a disclosure, and a
dismissed thread not shown at all — the transcript is where it left its trace.

The composer is a card in the sidecar, not a popover at the selection: the
bubble prevents mousedown precisely so it never takes focus, and every
script-positioned overlay in this repo has cost a bug. The passage is captured
inside the click that opens the draft, because focusing the composer destroys
the selection that summoned it.

comment:start no longer sends block digests. A client cannot compute a
canonical block digest without re-serialising the document, and finding the
quote is the better check anyway — it tests whether the sentence somebody
selected is still there. Driving the real socket showed it refusing stale
indices exactly as intended.

Also from driving it: Chat.instruct opened an agent session under AGENT=off,
where chat:send is gated but a button press was not. The decision is still
recorded; only the turn is skipped, and the transcript says so.