Add shared GLOSSARY.md for PM framework definitions

[priyanshuj-ai]

Terms like RICE, INVEST, MoSCoW, Kano, and Pyramid Principle are used across multiple skills but never defined in one place. While experienced PMs know these, a shared glossary would:

1. Help PM newcomers understand the frameworks without leaving the tool
2. Serve as a quick reference during prioritization sessions
3. Reduce the need for each skill to re-explain shared concepts

Suggested implementation: Create shared/glossary.md with one-paragraph definitions and links to canonical sources.