docs: add the Cratis ecosystem section and tighten grounded-answers wording

[woksin]

Part of the org-wide discoverability pass.

• Adds the canonical Cratis ecosystem section so readers landing on Prompter can find Chronicle, the language clients, Arc, Components, CLI + Workbench, the experimental model-first layer, supporting tools, and Samples.
• Rewords the intro's absolute "No hallucinated APIs" line into design-intent framing (grounded answers with citations, honest refusal) — no infallibility claims.
• Removes the third-party benchmark sentence from the retrieval description.
• Fixes a stray double period.

README-only; no code or behavior changes.