fix(cli): improve missing build output error message

[Mahakbajpai]

Summary

Improves the CLI error message shown when the build output (dist/entry.js) is missing during onboarding.
Instead of throwing a generic error, the CLI now provides a clear explanation and actionable steps to fix the issue.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactoring (no functional changes)

Related Issues

Fixes #7

Changes Made

• Replaced generic thrown error with a user-friendly error message
• Clearly explains that build output is missing
• Suggests running pnpm install and pnpm build
• Exits with proper status code after displaying guidance

Checklist

• My code follows the project's coding style
• I have run pnpm check and pnpm test locally
• I have updated documentation if needed
• My changes generate no new warnings or errors

[github-actions]

👋 Welcome to OpenSoul! Thanks for your first pull request.

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• You've run pnpm check locally and it passes
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[NJX-njx]

@Mahakbajpai Thank you very much, and you are welcome to continue making some interesting contributions