Migrate shell --live task selector from survey to huh

[blsmth]

Summary

• Migrate shell --live task selector from survey.Ask to huh.NewSelect[int]
• Extract ShellTaskSelectForm as a pure form builder function
• Add teatest TUI tests and testscript CLI tests
• Add VHS recording for visual verification

Closes #117

Demo

Changes

File	Purpose
cmd/shell.go	Replace survey with huh; extract ShellTaskSelectForm
cmd/shell_test.go	teatest tests (select first, second, last)
testdata/script/shell.txtar	testscript for help output and missing flag error
demo-shell-live.tape	VHS script for recording the demo GIF
demo-shell-live.gif	Recorded demo

[blsmth]

@ipmb — Want to check if you're OK with us self-approving these survey-to-huh migration PRs to keep the project moving. Happy to wait for your review if you'd prefer, but wanted to flag it since these have been open a while.