Migrate cmd/shell.go from survey to huh

[blsmth]

Summary

Migrate the shell task selector in cmd/shell.go from survey to huh.

Details

• Complexity: Low
• Prompts: 1 Select (task picker for --live flag)
• Direct survey.Ask() call (not via ui.AskQuestions)
• Similar to the scheduledTasks pattern from PR Migrate scheduled-tasks delete from survey to huh #116 — use Select[int] with task index as value
• Conditional on --live flag

Checklist

• Extract form builder function (pure, no AWS mocks needed)
• Replace survey.Ask() with huh form
• Add teatest + ui/uitest tests
• Add VHS recording for visual verification
• Verify non-interactive mode still works