shellui migrate for template API changes #58

Summary

When a template's API changes (for example Tabs moving from an items list to composition), shellui update replaces the component but leaves the app's pages using the old API, so the build breaks.

Proposal

shellui migrate <component> rewrites known patterns in the app's .razor files for a given change, shows a diff, and asks before writing. Start with the changes listed in the release notes' upgrade sections.