enable: address PR review — make the picker-path test meaningful

Route the real interactive multi-select through a package-level promptAgentSelection seam (matching runDispatchWizardForm/runTrailCreateForm) and collapse the selectFn-vs-form branch into a single selection step, so a lone detected agent has no "skip the picker" path to slip back into.

Rewrite the regression test to stub that seam and drive the selectFn == nil path, so it now fails if the single-built-in auto-use shortcut returns — the previous test injected a selectFn and would have passed against the old code.

Also clarify the doc comment that selectFn/--yes bypasses the prompt even on a TTY.