fix(update): route the self-update by install kind (apt/pip/source) — 0.37.1

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>

CHANGELOG.md

@@ -5,6 +5,14 @@ All notable changes to RigDoctor are recorded here. Format follows
 (`MAJOR.MINOR.PATCH`, pre-1.0). `__version__` and `pyproject.toml` must match the git
 release tag (so the auto-updater, D18, can compare versions).
 
+## [0.37.1] - 2026-05-22
+### Fixed
+- **`rigdoctor update` now uses the right method for how RigDoctor was installed.** It detects
+  apt (`.deb`), pip (venv/`.run`), or source installs (`updates.install_kind()`); only pip
+  installs self-update in place. An apt install no longer fails with "No module named pip" —
+  it (and the GUI Update button) shows `sudo apt update && sudo apt install --only-upgrade
+  rigdoctor`; a source checkout points to `git pull`.
+
 ## [0.37.0] - 2026-05-22
 ### Added
 - **Version footer** — a footer across the bottom of the window shows `RigDoctor v<version>` in