clean: remove scaffolding artifacts and update installer

Remove demo files (flappy bird, calculator, hello, jumping ball,
architecture diagrams) that were created during agent scaffolding.
Update install.sh with sprite banner, quick/advanced setup modes,
and auto browser launch.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: shreyas-lyzr