docs: add quickstart troubleshooting notes

[dashitongzhi]

Summary

• Add a Quick Start validation checklist for confirming a generated Agent-Native app boots correctly.
• Document common first-run failures around Node, pnpm, stale installs, ports, credentials, database setup, and browser-only errors.
• Include a clean retry path for partially generated apps.

Test Plan

• git diff --check

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[builder-io-integration]

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Review Details

This PR adds a Quick Start validation checklist plus first-run troubleshooting guidance to README.md. The overall approach is good: it gives new users a concrete smoke test after pnpm dev and a clean retry path instead of encouraging ad hoc edits to partially generated apps.

Risk assessment: low. The diff is documentation-only, but the new troubleshooting bullets need to match scaffolded project behavior exactly because they will be used when users are already blocked. I found two accuracy issues in that guidance: 🟡 the Node-version bullet points readers at package.json, even though some generated workspace roots do not declare engines.node and the repo also carries a separate .nvmrc; 🟡 the credentials bullet says to check a generated .env file, but common scaffold paths ship .env.example and require users to create .env themselves. Aside from those details, the validation checklist itself is practical and well-targeted.

🧪 Browser testing: Skipped — PR only modifies backend/config/docs, no UI impact

[steve8708]

thanks @dashitongzhi! I'm not sure right on the main README is the best place for this. maybe add a separate troubleshooting doc and we can link to it from the readme?