Add GitHub Actions provider foundation (Phase 1)

This commit implements the basic infrastructure for generating GitHub Actions
workflows from cigen configuration, establishing the foundation for full
GitHub Actions support.

Changes:

**Documentation:**
- Add comprehensive plan document: notes/SELF_HOSTING_ON_GITHUB_ACTIONS.md
 - 7-phase implementation roadmap
 - Architecture comparison: CircleCI vs GitHub Actions
 - Job skipping strategy (early-exit pattern for GHA)
 - Self-hosted runner support design
 - Dual-provider CI strategy (run on both CircleCI and GitHub Actions)

**GitHub Actions Provider Implementation:**
- src/providers/github_actions/schema.rs: Complete workflow schema
 - Workflow, Job, Step structures
 - RunsOn (single/multiple/detailed with groups/labels)
 - Strategy (matrix builds)
 - Service containers
 - Full step types (run, uses, with parameters)

- src/providers/github_actions/generator.rs: Basic workflow generator
 - Convert cigen Job → GitHub Actions Job
 - Parse steps from YAML values
 - Auto-inject checkout step
 - Convert job requirements → needs (AND dependencies)
 - Handle CheckoutSetting (Bool/Config)

- src/providers/github_actions/mod.rs: Provider implementation
 - Implements Provider trait
 - Default output path: .github/workflows
 - generate_workflow() and generate_all() support

- src/providers/github_actions/tests.rs: Test coverage
 - Provider name and output path tests
 - Workflow serialization test
 - Basic workflow generation integration test

- src/providers/mod.rs: Register github-actions provider
 - Add to get_provider() match statement

**Test Results:**
- ✅ All tests pass (4 new tests)
- ✅ Clippy clean (-D warnings)

**What Works:**
- Basic workflow generation from cigen config
- Job dependencies (needs)
- Simple run steps
- Checkout step injection
- YAML serialization

**TODO (Future Phases):**
- Phase 2: Conditionals, matrices, services, environment variables
- Phase 3: Intelligent caching with version detection
- Phase 4: Job skipping (early-exit with hash checking)
- Phase 5: Self-hosted runners with custom cache backends
- Phase 6: Automatic package management
- Phase 7: CIGen self-hosting on both providers

This is the first step toward "write once, run anywhere" - the same cigen
config will work on both CircleCI and GitHub Actions.

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

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

Author: ndbroadbent