Fix notebook schema validation errors

[ThierryHJ]

Summary

• remove invalid code-only fields from markdown cells in affected template and solution notebooks
• add a notebook validation script with --fix support
• add a GitHub Actions workflow to validate notebooks on PRs and master pushes

Root cause

GitHub's notebook renderer rejects notebooks where markdown cells contain code-cell-only fields like outputs. The causal-attention notebook had outputs: [] on its first markdown cell, and the same schema issue appeared in 66 notebooks.

Validation

• python3 scripts/validate_notebooks.py
• python3 -c 'import nbformat; nb=nbformat.read("templates/09_causal_attention.ipynb", as_version=4); nbformat.validate(nb); print("templates/09_causal_attention.ipynb valid")'
• env PYTHONPYCACHEPREFIX=/private/tmp/torchcode-pycache python3 -m py_compile scripts/validate_notebooks.py
• git diff --check

[ThierryHJ]

Closing for now because the remaining notebook preview failure appears to be a GitHub renderer issue affecting other repositories too. The branch keeps the local notebook hygiene fixes, and we can reopen or recreate this PR after GitHub resolves the global renderer issue.