docs: rewrite the execution guide — every financial procedure, from the screens

Ten scenarios in click order, covering what the review will actually ask:
splitting a fee across funds, connecting an unconnected charge (and declaring a
new source live), the rent cycle end to end, the cheque lifecycle including
bounce and re-presentation, expenses, restructuring the chart, VAT, deferred
revenue, breaking out an aggregate, and proving the numbers reconcile.

The closing line is the point: when asked about something not listed, the answer
is no longer "that needs development" — it is "we define it as a billing source
or a voucher type, let's do it now".
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>