docx skill helper scripts need python3 on macOS

When editing an existing Word .docx via the unzip/edit/rezip workflow, the skill helper scripts (merge_runs.py, validate.py) are documented as bare python which fails with command not found on macOS where only python3 is present; invoke them with python3. Run merge_runs.py before searching document.xml because Word fragments a visible phrase across multiple <w:r> runs, so the contiguous string (e.g. a full name) may not exist until adjacent runs are merged.