Remove bash execution from cookbook notebooks

[wavebyrd]

Summary

• Remove !pip install commands from all 5 cookbook notebooks
• Remove git clone and os.chdir() commands that were used for Colab/Kaggle setup
• Replace with simple Python-based directory detection

Changes

All 5 notebooks updated:

• 1-RiskReturnModels.ipynb
• 2-Mean-Variance-Optimisation.ipynb
• 3-Advanced-Mean-Variance-Optimisation.ipynb
• 4-Black-Litterman-Allocation.ipynb
• 5-Hierarchical-Risk-Parity.ipynb

The preamble cell now just handles the directory structure:

import os

# Change to cookbook directory if running from repo root
if os.path.isdir("cookbook/data") and not os.path.isdir("data"):
    os.chdir("cookbook")

Test plan

• Verify notebooks run locally with the notebooks depset
• Confirm no bash execution remains (!pip, !git, %%bash)

Fixes #717

🤖 Generated with Claude Code

[wavebyrd]

Ping on this notebook cleanup - removes bash execution from cookbook. Ready for review!