[Jammy2211]

Summary

Before this fix, autohands generate <project> deleted the tracked root start_here*.ipynb before calling ipynb-py-convert. When that converter wasn't on PATH (it is missing from the PyAuto313 venv), the run failed partway, leaving start_here.ipynb deleted and a stray temp.py. This happened to 3 workspaces on 2026-11-04.

• Preflight check: generate.py now checks shutil.which("ipynb-py-convert") immediately after the unknown-project guard. If the converter is missing, it exits non-zero with an install hint and "Nothing was modified".
• Stale-assistant warning: a new build_util.assistant_behind_origin(root) counts how far the assistant checkout is behind origin/main, using local refs only and never fetching. generate.py warns, without stopping, when it is behind. A stale assistant silently rewrote the Gemini context of 239 notebooks in the same session.
• The converter name is now one constant, build_util.NOTEBOOK_CONVERTER.

Closes #330.

Tests

• test_missing_converter_exits_nonzero_without_touching_anything runs generate with a PATH that has no converter. It asserts a non-zero exit, a clean git status, start_here.ipynb still present, and no temp.py. Control check: it fails against the old generate.py.
• test_assistant_behind_origin_* covers lag counting and the non-git case.
• The full suite gives 587 passed and 4 failed. The 4 failures are test_tombstone.py sdist-build tests, which fail identically on unmodified main, so they're unrelated.

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