fix Skills public imports

[zyz254009-crypto]

What changed

• Fix all 13 skills.<category> package entry points to use their real relative subpackages instead of the missing office.skills namespace.
• Normalize 73 Skill documents and the Skills index to show the working from skills.<category> import ... import path.
• Add an import regression test covering 13 categories and all 73 public __all__ exports.
• Add a detailed Chinese optimization report under docs/skills-import-optimization-report.md.

Why

The main README recommends imports such as from skills.pdf import pdf2docx, but category __init__.py files imported from a non-existent office.skills package. Following the documented quick start therefore raised ModuleNotFoundError: No module named 'office.skills'.

Impact

The documented Skills imports now work consistently without changing the underlying office.api business implementations. This also provides a stable public discovery surface for future desktop-client integration.

Validation

• python -m unittest tests.test_code.test_skills_imports -v
• Verified 13 categories and 73 public exports are importable and callable.
• Verified common README imports for PDF, image, tools, and Excel.
• Verified no stale office.skills references remain under skills.
• git diff --check

User-created local files demo.py and qrcode.png are intentionally excluded.