Typing `/model` and pressing Enter did nothing in the TUI; the picker
only opened via `/model --save` or `/model <name>`. The completion popup
matched both `/model` and `/model-fallback` by prefix, so the
exact-match-hide guard (which required a single match) never fired. With
the popup still visible, the TUI's Enter handler completed the text
instead of submitting the command, so it never executed.
Treat the typed prefix as an exact match whenever it equals any matched
command name, not only when it is the sole match. This hides the popup
on a complete command name so Enter submits it, even when a longer
command shares the prefix.
Ziheng Zhang
6593ac9b5b