cronjob(action='run', prompt=...) context was silently dropped when the
manual run forwarded to the gateway (#96010 follow-up): POST
/api/jobs/{id}/run took no body. The forward now sends {prompt} in the
request body; the api_server validates it (length cap + strict injection
scan, same as stored prompts) and trigger_job stamps it as a transient
manual_run_prompt alongside manual_run_at. run_one_job consumes the stamp
for that single fire and mark_job_run clears it, so it never persists
into the job definition or later scheduled fires.
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