Add beginner-friendly RAG + semcode presentation (excalidraw)

Adds a 22-slide .excalidraw deck covering RAG fundamentals (embeddings,
vector search, dense vs sparse) before diving into how semcode's
ingestion/retrieval pipeline and MCP prompts differ from traditional
prose RAG.

Closes #87

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: GoodbyePlanet