feat(keyword-detector): handle ultrawork keyword after greeting patterns

- Add TRAILING_GREETING_ULTRAWORK_PATTERN to detect 'hi ultrawork' style inputs
- Rename hasLeadingUltraworkKeyword to hasEdgeUltraworkKeyword for clarity
- Add extractUltraworkTask guard to return empty string for greeting-only inputs
- Add comprehensive test coverage for edge trigger scenarios

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>