docs: carry the item 20 and 53 backlog entries onto master

The work for item 20 lives on the branch item-20-message-rows, parked and not
merged while the user explores a layout other than the table. Without these
documents on master the session-start backlog read would miss item 53 entirely
and item 20 would still read as unspecified, so the next session would either
re-ask a question already answered or start designing against a rejected design.

Docs only. No code from the branch is merged here, and the entries say so in
three places: a note above the status table, item 20's own section, and a banner
on the plan document telling a future agent not to execute it again.

File and line references were checked against master rather than copied. Item
53's citation of the column-grid cause pointed at threadlistmodel.cpp:275-283,
which on master is unrelated font handling; it now names the branch and the
function instead of a line range that would send a reader to the wrong code.

CLAUDE.md and CHANGELOG.md were deliberately NOT brought across. Both describe a
QTreeView and features master does not have, and a CLAUDE.md that misdescribes
the architecture is worse than one that is merely out of date.

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