feat: implement GitHub Actions cache generation (Phase 3)

Implemented automatic cache system for GitHub Actions that mirrors the
CircleCI caching functionality with platform-specific optimizations.

Key features:
- Automatic cache restoration from job.cache and job.restore_cache fields
- Template conversion from CircleCI format to GitHub Actions format
 ({{ checksum "file" }} → ${{ hashFiles('file') }})
- Smart fallback keys for cache restoration
- Uses actions/cache@v4 with automatic save on success
- Full test coverage with 4 test cases

Implementation:
- Created src/providers/github_actions/cache.rs with CacheGenerator
- Integrated cache step injection into workflow generator
- Cache steps are inserted after checkout, before user steps
- Supports both path-based and definition-based cache configurations

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ndbroadbent