feat: generate separate workflow files based on .cigen/workflows/ structure

- Added 'workflow' field to Job schema to track which workflow a job belongs to
- Updated loader to set workflow field based on directory structure
- Added workflow field to protobuf JobDefinition
- Updated GitHub Actions plugin to group jobs by workflow and generate separate files
- Now generates .github/workflows/ci.yml and docs.yml properly
- NOTE: release.yml not yet working - needs matrix support and workflow metadata reading

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

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

Author: ndbroadbent