macOS `system` step should show available updates

## Checklist - [x] I have searched the issue tracker for relevant or duplicate issues. ## I want to suggest some general feature Currently: ```console $ topgrade --only system ── 00:00:00 - macOS system update ────────────────────────────────────────────── Finding available software A system update is available. Do you wish to install it? (Y)es/(N)o ── 00:00:00 - Summary ────────────────────────────────────────────────────────── ``` It should list the available updates. Some might consider not listing the available updates a bug. e.g. `softwareupdate -l` outputs: ```console $ softwareupdate -l Software Update Tool Finding available software Software Update found the following new or updated software: * Label: macOS Tahoe 26.5.2-25F84 Title: macOS Tahoe 26.5.2, Version: 26.5.2, Size: 2630358KiB, Recommended: YES, Action: restart, ``` It would also be nice to be able to multi-select individual updates (via displayed numbers; I assume you'd need to parse & reformat `softwareupdate -l` output), (a)ll, or (n)one to install. ## More information <!-- Assuming that someone else implements the feature, please state if you know how to test it from a side branch of Topgrade. --> I should be able to figure out how to test, as long as I have some pending uninstalled system software update, but I try to install such updates ASAP. - [ ] I am able and willing to implement this feature myself I don't know the topgrade codebase, and am probably too busy with other stuff to learn it now.