Agent-only wallet accounts top up, watch their credit and recover a lost key

On an account an agent opened with a wallet and no person pays for, an out-of-credit answer now tells the agent to add credit itself with npx withruntime billing topup <usd> (details.addCredit.by is agent). GET /v1/limits gains credit: the balance, the daily pace, the days it lasts and whether it is ok, low or empty. Webhooks can subscribe to credit.low and credit.empty. A lost key is replaced with the claim code (runtime billing claim <code> --new-key), which revokes every other key; at most 3 times a day, logged in the audit log, until a person takes the account over. See pay with stablecoins, limits and events.