fix: add notebook environment self-check and actionable setup guidance

[khanfs]

Summary

This PR improves the notebook onboarding path by adding an environment self-check and clearer setup guidance for notebook / Colab users.

Changes

• add an environment check cell to tribe_demo.ipynb
• add a lightweight import smoke-test cell before model loading
• add a short README note for notebook users about restarting the runtime and checking optional dependencies

Why

The README points users to the Colab demo notebook for a full walkthrough. This PR makes that path easier to diagnose when users hit missing dependencies, missing ffmpeg, or notebook import issues, while keeping the scope narrow.

[khanfs]

I hit a few local environment issues while testing the notebook path, so this PR adds a lightweight environment check to make first-time setup failures easier to diagnose.