[NA] [DOCS] README: add Opik entity line and Claude Code toolkit cross-links

[Brianwones]

Details

Context for reviewers: This PR is part of an approved marketing project tracked in the Marketing Experiments Dashboard in Notion ("GitHub org GEO: one identity across 22 repos"). AI assistants currently describe Opik, from memory, as "Comet's experiment tracker" (the wrong category), and the fix is one consistent description of Opik repeated across our public READMEs so AI models and search engines learn the right answer. If the wording looks deliberately uniform with other repos' PRs, that is intentional; please flag anything factually wrong, but keep the shared sentences verbatim where you can. README/metadata text only, no code changes. Questions: Brian Wones (Collimer) or the marketing team.

Adds the canonical Opik description after the opening paragraph, and an "Opik for Claude Code" cross-link section above the License notice. The existing prose is untouched.

Change checklist

• User facing
• Documentation update

Issues

• Resolves #

Testing

Documentation-only change (no code paths). Verified: added links resolve (200); text matches the repo's existing style; no other references affected.

Documentation

This PR is the documentation change.

AI-WATERMARK

AI-WATERMARK: yes

• Tools: Claude (Anthropic)
• Scope: README documentation text only. No code, no product behavior changed. Every added link verified to resolve on 2026-07-23.