docs: reconcile the backlog and close item 184

Three entries in the user's notes had no item here, all verified in the code
rather than copied from the note:

- 198, the unsynced-changes list naming no account. PendingChangeRow carries
  subject, action, startsMessage and messageCount and nothing else, so changes
  across five accounts draw as one undifferentiated run. The data is reachable:
  accountForMessagePath() answers from a path and resolvePendingSubjects()
  already crosses to the worker with every id. Held thread edits are keyed on a
  thread id rather than a message id and need their own answer first.
- 199, shipping our own chrome icons. This reverses the split item 70 chose
  deliberately, panes ours and chrome the system's, so it is a decision to
  revisit rather than a defect. The mechanism is proven by Marks; the artwork
  is the item, roughly forty actions against item 70's six marks.
- 200, launching at an account, thread or message. main.cpp recognises only
  --version and --help, both answering before QApplication exists. There is no
  single-instance mechanism anywhere in src/.

Item 184 closes, built outside this repository as mail-watcher and confirmed
running on this machine. It took the shape the entry argued for: a watcher of
ours rather than a third-party daemon, one Python file on the standard library,
the cron tick kept as a backstop and /tmp/mbsync.lock still the shared mutex.
Nothing in src/ changed, which was the point. Its section moves to the closed
file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01P3HQXLauwQgzxR4YfJBB3x

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