docs: fold deferred findings into the task 13 checklist

Adds two checks the earlier tasks turned up but only recorded in passing:

- Item 21: config.cpp validates the sync command by splitting on a space,
  while MailSync splits it with QProcess::splitCommand. A quoted program
  path containing a space makes the two disagree. The failure is safe
  (sync disables itself) but the message misleads, so the check is to
  confirm the behaviour and then pick a side.
- Item 22: deleting the sync script while running exercises the launch
  failure path, which config's load-time validation cannot cover.

Also corrects two claims the plan had outgrown: task 13 is no longer the
notmuch layer's only coverage now that task 8 tests against a throwaway
database, and cron runs every 10 minutes rather than hourly.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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