Add beginner-friendly RAG + semcode presentation (excalidraw)

[GoodbyePlanet]

Summary

• Adds presentation/semcode-rag.excalidraw, a 22-slide deck importable at excalidraw.com (File → Open)
• Part 1 (slides 1-7): RAG fundamentals for beginners — what RAG is, embeddings, vector DBs, dense vs sparse vectors, generic pipeline
• Part 2 (8-9): why code needs a different RAG than prose, with a traditional-vs-semcode comparison table
• Part 3 (10-15): semcode's ingestion pipeline — Tree-sitter → CodeSymbol → dense/sparse embedding text construction → incremental blob-SHA indexing
• Part 4 (16-18): stored point anatomy, payload indexes, hybrid retrieval + RRF worked example
• Part 5 (19-21): MCP tools overview, what an MCP "prompt" is, and a walkthrough of service_overview/system_design_overview
• Slide 22: recap diagram + takeaways

Closes #87

Test plan

• Validated the JSON is well-formed and all 453 elements have unique ids
• Verified no text elements overflow their frame bounds
• Open presentation/semcode-rag.excalidraw at excalidraw.com and visually page through all 22 slides

🤖 Generated with Claude Code