Add research-paper-translation to .experimental

[Nickscooo]

Summary

• add research-paper-translation under skills/.experimental
• package a version-aware workflow for paragraph-aligned Chinese translation of research paper PDFs
• include a small extraction helper and reference guidance for extraction fallbacks

Why this belongs in .experimental

This skill is robust and reusable, but it is still a focused workflow for Chinese close reading of research papers, so .experimental feels like the right first category.

What it does

• lock to the exact PDF artifact the user provides
• prefer real PDF extraction before translation
• translate paragraph by paragraph instead of summarizing
• preserve numbers, model names, datasets, and metrics
• help users verify whether an existing translation matches the source paper