ci: add comprehensive PR validation workflow for all SDK demos

[hattori7243]

Summary

Introduces a comprehensive, parallel GitHub Actions CI workflow (.github/workflows/ci.yml) to automatically validate all SDK demos on pull requests and pushes to main.

What it validates

1. Catalog & Script Tests:
   • node scripts/verify-catalog.mjs (Validates catalog consistency across examples.json and README.md)
   • node --test scripts/*.test.mjs (Pub lock parser tests)
2. TypeScript Demos:
   • Browser key-file runner unit tests
   • Electron key-file runner unit tests
   • Full Vite build & subpath-safe asset verification for Browser demo
   • Full Vite + Electron build & asset verification for Electron demo
3. Flutter Runner Scripts:
   • iOS/Android release and debug runner script unit tests
4. Python Voice Agent Demo:
   • Runs 35 pytest unit tests under isolated uv environment with cached dependencies