Update module docs: fix arXiv URL, uv setup, nanoGPT clone path

- Use HTTPS for arXiv API (was returning 301 on HTTP)
- Point module 01 preliminaries to root uv sync instead of separate venv
- Clone nanoGPT into 01-nanogpt/ and add to .gitignore
- Add llama3.1:8B to module 02 models table
- Various editorial updates to modules 01 and 02

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -33,6 +33,9 @@ store/
 # Personal notes (not part of the workshop)
 *-notes.md
 
+# Cloned repositories
+01-nanogpt/nanoGPT/
+
 # Legacy directories (not part of the workshop)
 handouts/
 class_demo/