CLI: V3 by Default, Classic and Desktop App Retired

This release makes V3 the default agent harness for interactive Kiro CLI sessions on hosts that can run it. Existing V2 users are asked before the default applies, and Kiro upgrades custom agents still in the V2 format unless you turn that off. Kiro CLI is now a single standalone kiro-cli binary that retires the Classic interface, the desktop app, and inline autocomplete.