Fix the free-tier notes after checking each service's own pages

[eugeneshilow]

I checked the pages of the five services on 2026-10-01 and my notes were wrong in two places.

• Cerebras is not free any more: it gives a $5 trial for 30 days and asks for a card. It is now marked optional.
• Mistral has no Experiment plan. Its free plan is $10 a month in API credits, no card.
• Groq's free plan allows 8K tokens a minute, so big files hit the limit quickly.
• Free tiers may use what you send to improve their models. The README and anus setup now say so.

Tests: 28 pass.