feat: add setup subaction with caching and plugin support

[amondnet]

• Add standalone setup/ subaction for Claude Code installation
• Support version pinning (stable, latest, specific versions)
• Implement caching with daily rotation for latest/stable
• Add plugin marketplace and installation support
• Include security validation from base-action patterns
• Add comprehensive unit tests (29 tests, 85 assertions)
• Update CI with matrix strategy for parallel testing
• Add integration test workflow for setup action