Jupyter compatibility (.ipynb import/export)

[rgbkrk]

Implement Jupyter notebook compatibility with seamless .ipynb import/export.

Current State: Anode-specific notebook format
Target: Full .ipynb compatibility for import/export

Implementation Tasks:

• .ipynb import functionality - Convert Jupyter notebooks to Anode format
• .ipynb export functionality - Convert Anode notebooks to Jupyter format
• Metadata preservation - Maintain notebook metadata across conversions
• Cell type mapping - Handle all Jupyter cell types (code, markdown, raw)
• Output format compatibility - Convert outputs between formats

Features:

• Import existing Jupyter notebooks
• Export notebooks for sharing/backup
• Metadata preservation
• Cell type compatibility
• Output format conversion

TBD: Metadata mapping strategy, output format conversion, cell type handling

[rgbkrk]

Export done in #620.