Story 22.2 - Import notebook files

[acbart]

As a learner, I want to import notebook files so that I can reuse work from Jupyter.

Acceptance criteria:

• .ipynb files are parsed as JSON.
• Code cells are included unless the first source line starts with %.
• Markdown and raw cells are converted into triple-quoted Python strings.
• Invalid notebooks show an import error and preserve current code.
• Import result can be run after conversion.