docs: keep the card list off master, and name the way back

The user asked to be able to abandon this cleanly. Task 0 now branches
`card-list` off a rebased copy rather than rebasing `item-20-message-rows` in
place, which would have destroyed the record of what the rejected presentation
looked like.

Nothing in the plan touches master, stated as a requirement rather than left to
convention, with the escape hatch written out: `git checkout master` is the
whole of the undo. Task 0 gains a step that verifies master and the original
branch are both where they started, since a rebase onto the wrong branch is
silent and every later task assumes master is the thing to fall back to.

Task 10 now ends by explicitly forbidding a merge. This design was reached by
rejecting a previous one that was finished, tested and green, so a passing suite
is not what decides it: the user looks at the cards. That is the one instruction
most likely to be lost between sessions, and it is the one that matters.

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