v2.64.3 — board agent: empty-prompt Run feedback

Fix: clicking Run with an empty prompt field (the placeholder reads like text) used to silently focus the field and do nothing — which read as 'the Run button doesn't work'. It now shows '✗ Enter a prompt first …' and flashes the field red.

To run an agent: type a prompt (e.g. /execute-plan or 'claim the next ready task and implement it') then Run.