docs(proxy): record the spend-vs-savings rule for local learn

Write down the conditions under which `caveman learn` may state a currency, so
the distinction survives the next person to touch this code: spend is arithmetic
over provider-counted tokens at dated published rates, savings additionally
requires a named attribution rung, totals never blend rungs, and neither surface
ever writes `verified`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>