rigdoctor update assumed a pip/venv install and ran 'python -m pip install', which
fails on a .deb (system python has no pip; you can't pip-upgrade a dpkg package).
Add updates.install_kind() (dpkg ownership / venv / source-checkout detection,
cached) and route apply_update: pip self-updates in place; apt and source installs
return guidance instead. CLI and the GUI Update button show the apt/git command.
Adds tests/test_updates.py.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
