_deliver_result() now returns Optional[str] — None on success, error
message on failure. All failure paths (unknown platform, platform
disabled, config load error, send failure, unresolvable target)
return descriptive error strings.
mark_job_run() gains delivery_error param, tracked as
last_delivery_error on the job — separate from agent execution errors.
A job where the agent succeeded but delivery failed shows
last_status='ok' + last_delivery_error='...'.
The cronjob list tool now surfaces last_delivery_error so agents and
users can see when cron outputs aren't arriving.
Inspired by PR #5863 (oxngon) — reimplemented with proper wiring.
Tests: 3 new mark_job_run tests + 6 new _deliver_result return tests.
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