docs(backlog): record item 44 as done

The tagging rules moved from the shell post-new hook to a shared JSON
store both qtmaildir and mailctl read. Seventeen real rules were
converted, each keeping its shell comment as a note, and the conversion
was proved against the real index before anything was installed.

Four findings are recorded in CLAUDE.md rather than only here, because
they will outlive the item: a stored query carries no scope and the hook
parenthesises it (a disjunction would otherwise escape tag:new and match
everything); notmuch's parser rejects almost nothing, so a test asserting
a provoked query failure fails against correct code; rule counts must
count messages rather than threads; and a count request must not bump the
query generation, which would blank the message pane.

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