[fix]: fix cursor deep link, add readme clarification for model apikey

[Kylejeong2]

what

made readme more obvious + added updated cursor deeplink

[greptile-apps]

Greptile Summary

This PR makes two significant documentation improvements to the README.md file:

1. Updates the Cursor deep link configuration to properly include the GEMINI_API_KEY environment variable and fixes the command format for NPX installation.
2. Adds a prominent notice about model API key requirements, making it immediately clear to users that different models require specific API keys.

These changes enhance the user onboarding experience by making requirements more transparent and fixing installation tooling.

Confidence score: 5/5

1. This PR is extremely safe to merge as it only modifies documentation with accurate information
2. The changes are well-documented, technically accurate, and improve user experience without touching any functional code
3. No files need special attention as this is a straightforward documentation update

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