docs: add product eval adoption guide

[drewstone]

Summary

• tighten README around real product-loop adoption
• add product eval adoption guide covering adapters, validators, traces, feedback trajectories, datasets, optimization, and release gates
• document deterministic checks before judges and product-owned boundaries

Verification

• pnpm typecheck
• pnpm test
• pnpm build

[drewstone]

Published @tangle-network/agent-eval@0.20.11 after local verification. npm latest now resolves to 0.20.11.