ci: Optimize GitHub Actions with sccache and parallel test matrix

Optimizations:
- Add sccache for faster incremental builds (up to 10x faster)
- Use test matrix for parallel unit/e2e/ovsm tests
- Add concurrency control to cancel redundant runs
- Add OVSM script validation job for example files
- Consolidate ci.yml and ci-optimized.yml into single workflow
- Remove duplicate ci-optimized.yml

New features:
- OVSM script validation catches syntax errors before merge
- Parallel test execution reduces wall-clock time
- sccache persists across runs for faster builds

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: 0xrinegade