Add NotebookEdit tool for Jupyter notebook cell editing

[evansenter]

Summary

Add tool for editing individual cells in Jupyter notebooks (.ipynb files).

Proposed API

{
  "notebook_path": "string",
  "cell_id": "string (optional)",
  "cell_type": "code | markdown",
  "new_source": "string",
  "edit_mode": "replace | insert | delete"
}

Operations

• replace: Replace cell content
• insert: Add new cell after specified cell_id
• delete: Remove cell

Dependencies

Requires #15 (read_file notebook support) to be useful.

Complexity

Medium - need to parse/modify ipynb JSON structure while preserving metadata.