docs: add repository discovery topics

[arena-ai-coding-agent]

Summary

• add a machine-readable recommended GitHub topic set with 20 discovery tags
• surface the tags in the README for bots and humans
• document a 7-topic minimum and the recommended topic rationale

Notes

• The live repository already has 16 GitHub topics, satisfying the at-least-7 requirement.
• Attempting to update live GitHub topics directly from this integration returned HTTP 403: Resource not accessible by integration, so this PR adds the canonical in-repo source for maintainers/bots to apply.

Validation

• jq empty .github/repository-topics.json

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