exec: add --output flag to write executed notebook to file

[anish749]

Problem

When executing a complete notebook via --file, there's no way to get the executed notebook with outputs as a file. The agent only sees streamed text output but can't point users to a rendered notebook.

Solution

Add --output option to nbexec exec for .ipynb files:

Scenario	Behavior
Full run + --output	Write executed notebook to specified path
Full run, no --output	Auto-generate <input>_out.ipynb, report path on stderr
Partial run + --output	Update only executed cells in output file (preserves prior partial run outputs)
Partial run, no --output	No output file (unchanged behavior)

• Output notebook preserves all cells (markdown, etc.) from the input
• Output is written even on cell failure so partial results are not lost
• --output is rejected for non-notebook files

Testing

8 new tests in TestOutputNotebook covering all scenarios above, including incremental partial runs and error handling. All 16 tests pass.