📖 Refresh Hive backend integrations docs

[clubanderson]

Summary

• Group Hive agent CLI backends by T1/T2/T3 support tier
• Add OpenHands as new experimental/headless-only and include current inference gateways
• Add related ecosystem positioning links for gh-aw, Symphony, Goose/AAIF, OpenHands, and vibe-kanban
• Add the official OpenHands SVG logo asset

Validation

• npm run build
• npx markdownlint-cli2 docs/content/hive/integrations.md

[kubestellar-prow]

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