feat(ci): smart build detection - only compile changed firmware

- Add detect-changes job to analyze file modifications
- Build IoTButtonV2_DeepSleep only when its example or library src changes
- Build WiFiProvisioning only when its example or library src changes
- Library source changes trigger ALL firmware rebuild
- Support manual trigger with build_all or specific firmware selection
- Parallel build when multiple firmwares need update

Author: limengdu