docs: add VHS demo tape for animated CLI showcase

[y0s3ph]

Summary

• Adds demo.tape — a VHS script that records the full gostrap workflow as an animated GIF
• Showcases: init wizard → add-app → validate → diff
• Uses Catppuccin Mocha theme for consistent, professional look

How to generate the GIF

go install github.com/charmbracelet/vhs@latest
go build -o gostrap ./cmd/gostrap/
export PATH="$PWD:$PATH"
vhs demo.tape

This produces demo.gif which can be embedded in the README:

<p align="center">
  <img src="demo.gif" alt="gostrap demo" width="700">
</p>

Why VHS?

• Same ecosystem as our CLI (Charmbracelet)
• The .tape file is version-controlled — if the wizard changes, update the tape and regenerate
• Deterministic, reproducible output
• Can be automated in CI