docs: add cloud Gemini agent setup recipes

[chiveswang]

Summary

• update the Gemini example model from the old experimental Flash model to gemini-2.5-flash
• document using cloud Gemini Flash / Flash-Lite models to reduce local hardware load
• add guidance for hosted Gemma 4 endpoints and when to use openai_compatible_llm
• add a reusable setup prompt for Claude, Codex, or another agent to safely edit local conf.yaml

Validation

• npm run build

Notes

• No API keys or private endpoint values are included.
• The rate-limit guidance points users to AI Studio because Gemini RPM/TPM/RPD limits vary by project, model, billing tier, and account state.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 91761d04-6028-4a25-b804-6736d5956e09

📥 Commits

Reviewing files that changed from the base of the PR and between 622a307 and b6a1634.

📒 Files selected for processing (2)

• docs/user-guide/backend/llm.md
• i18n/en/docusaurus-plugin-content-docs/current/user-guide/backend/llm.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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