Add routing guardrails for cost caps and provider fallback

[johnnyhuy]

Provider comparison notes suggest a practical gateway value: routing rules that cap cost and handle fallback cleanly instead of forcing every client to rebuild the logic.

Starter scope:

• provider fallback model
• max price or budget caps
• clear policy for default versus premium routes

Sources:

• Notes/2026-04-19 - OpenRouter vs GitHub Copilot vs direct providers.md
• Notes/2026-04-19 - OpenRouter vs GitHub Copilot Cost and Model Recommendation.md

Assignee: johnnyhuy