#14677: fix: strengthen CRON_EVENT_PROMPT to ensure agents execute tasks

## Problem The existing CRON_EVENT_PROMPT is too passive: > *A scheduled reminder has been triggered. Please relay this reminder to the user in a helpful and friendly way.* Agents interpret 'relay' as informational acknowledgment and dismiss cron systemEvent tasks without executing the actual work. ## Fix Strengthen the prompt to be explicit: > *A scheduled cron job has fired. The task instructions are in the system messages above. Execute the requested task fully — do NOT reply with HEARTBEAT_OK. Complete the work described, then report the results.* Matches the assertive tone of EXEC_EVENT_PROMPT which works reliably for exec completions. ## Notes - The cron detection wiring (reason prefix, peekSystemEvents, empty-heartbeat-file bypass) is already correct on main - Only the prompt text needs changing — 3 lines