fix: model routing bugs, per-model cost tracking, and quickstart doc#2

HelloAnner wants to merge 4 commits into

billion-token-one-task:v6-releasebillion-token-one-task/ClawOSS:v6-releasefrom

HelloAnner:fix/model-routing-and-cost-trackingHelloAnner/ClawOSS:fix/model-routing-and-cost-trackingCopy head branch name to clipboard