docs: add research proposal for git-native AI memory

Comprehensive academic research proposal covering:
- Git-native semantic memory architecture
- Progressive hydration (SUMMARY → FULL → FILES)
- Cognitive namespacing (10 memory categories)
- Hook-based automation for capture/injection

Includes 26 citations across:
- LLM memory systems (MemGPT, Mem0, A-Mem, HippoRAG)
- Human memory cognitive science
- RAG and retrieval systems
- Semantic search and embeddings

References prior cs-memory prototype work.

Author: zircote