Start Agent Batch

The agent over every row of a table: one agent run a row, its prompt (and urls) filled from the row's columns as {{column}}, its answer the row's new columns (schema). A few rows run at once (concurrency); each pays for what it uses up to maxCreditsPerRow, and the batch never spends more than rows x that cap, or the balance when that was less -- rows it cannot afford are skipped, free. Rows that cannot run (a bad URL, an empty prompt) are kept as invalid and never charged. 202 with next to poll; the webhook hears agent.batch.finished once.