enable: pre-select detected agents instead of skipping the picker

On first run, a single detected built-in agent was used automatically and the multi-select picker was skipped entirely, so the user had no chance to add more agents without re-running. Route that case through the same picker used by the multi-agent and re-run paths, where detected built-in agents are already pre-selected — the user can confirm with enter or tick more first.

The non-interactive (no-TTY) path is unchanged: it still uses the detected agents without prompting. --yes likewise keeps using the caller's selection.