harmony: restore RNOH ReactCommon C++ sources so a fresh clone builds ohpm dropped third-party/rn/ReactCommon (yoga/jsi/cxxreact/...) when extracting the RNOH HAR, so the native CMake/ninja build failed with "add_subdirectory ... not an existing directory". These 1320 sources were never tracked (only their build-output .o files were), so a clone could not build the HAP. Restored them from local-har/react-native-openharmony-0.82.29.har and committed so clone + change signing + DevEco Run works with no manual fixes. Also: - drop regenerated native build artifacts (entry/.cxx, entry/build) from tracking; they embed machine-specific absolute paths and are recreated by DevEco, so a clean clone build is more reliable without them - add packages/app-mobile/harmony/.gitignore to keep those out - add README.OpenHarmony_CN.md: OpenHarmony build/run guide + the ReactCommon fix and clone prerequisites (Node 22, Yarn 4.12, git-lfs, DevEco SDK) Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>