docs(backlog): specify item 44 as a shared tagging-rule store

Item 44 sat as "open, unspecified" because nothing in this application
applies rules at sync time, and the item could not be planned until it
was known whether such rules existed anywhere. They do: the notmuch
post-new hook holds hand-written `notmuch tag` lines scoped to tag:new,
carrying their reasoning in shell comments.

The design moves them to a tool-neutral JSON store that both qtmaildir
and mailctl read, with unknown fields preserved across a write by either
tool so neither owns the format. A rule carries no scope, so the same
rule serves the hook, a dry run and a future backfill.

Also in this pass:

- Item 61's cause is established, not open. It is the user's cron sync
  holding the mbsync lock: 0 failures in 30 runs with no lock held, 30
  in 30 with one held. The fix is item 38's existing seam applied across
  the suite. The document still said "not established" and proposed a
  load hypothesis that had already failed to reproduce.
- Item 74 records the first-start latency measured this session. The
  delay is the notmuch index paging in from disk, 5714 ms cold against
  154 ms warm for the same 4444-thread query, and is not addressable
  here. What it did expose is a real defect: the status bar holds
  "Searching..." for the whole walk while rows are already arriving.

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