ci: test CLI on Linux, Windows, and macOS

[krowvin]

Summary

• expand CLI pull request testing from Linux-only to Linux, Windows, and macOS
• retain Python 3.9 and 3.12 coverage on Linux
• run Python 3.12 on Windows and macOS
• keep all matrix jobs running after one platform fails so platform-specific results are visible together

Why

This follows up on Mitch's review comment in #246. The update command had been exercised locally on Windows and WSL, but Mitch encountered an error: externally-managed-environment failure in a native Linux virtual environment. That surfaced an important gap: behavior that looks correct in one local environment can still fail under another operating system's Python packaging rules.

Running the test suite on native GitHub-hosted Linux, Windows, and macOS runners should expose these cross-environment differences earlier in pull requests.

Test matrix

Operating system	Python
Linux	3.9
Linux	3.12
Windows	3.12
macOS	3.12

Python 3.9 is tested only on Linux to provide the closest practical GitHub-hosted approximation for continued T7 support without multiplying every Python version across every operating system.

Solaris is not included because GitHub does not provide a hosted Solaris runner. Supporting it would require separately managed infrastructure rather than a standard Actions runner.

Runtime impact

The four jobs run in parallel. Existing Linux jobs generally complete in under a minute, so this should increase total runner usage more than pull request wall-clock time. The matrix intentionally avoids the six-job full operating-system/Python cross-product.

Validation

• parsed the updated workflow YAML and confirmed four matrix entries
• repository pre-commit checks passed, including yamlfix
• poetry check passed
• installed the project and development dependencies natively on Windows
• CLI smoke test passed
• full Windows test suite passed: 235 tests