docs: record the one coupling between qtmaildir and mailctl

The two tools are independent except for ~/.config/mailrules/rules.json,
which has two independent implementations agreeing by test rather than by
shared code. That is the only way work here can break mailctl, so it now
has a named procedure: change both readers, bump the format version only
for a breaking change, run both suites, and verify the round trip by hand
since no automated test spans the repos.

Also records that the backlog covers the mail system rather than this
binary alone. Item 44 already shipped as commits in both repos and future
format work will too.

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