chore: initial project scaffold for quant trading learning

Sets up project structure with yfinance-based OHLCV fetcher for top
100 S&P companies, Jupyter notebook scaffold, and uv-managed deps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.py

@@ -0,0 +1,6 @@
+def main():
+    print("Hello from learn-trading!")
+
+
+if __name__ == "__main__":
+    main()