bug(gateway): cron job outputs injected as assistant role, breaking message alternation

[teknium1]

Cron job outputs in gateway sessions are appended as assistant role messages, creating two consecutive assistant messages. This violates the expected user/assistant alternation.

Found during session audit — JSONL session 20260319_052206_eee156a5 has messages 231-232 both as assistant role (agent response followed by cron report).

Cron outputs should be injected as a user or system message to maintain proper alternation. Two consecutive assistant messages can confuse models and cause API rejections on strict providers.

[ygd58]

Submitted a fix in PR #2223. The root cause was in gateway/mirror.py cron delivery messages were being appended with role: "assistant", creating two consecutive assistant messages after the agent's response.

Fixed by changing to role: "user" and adding a [Cron delivery from {source_label}] prefix so the model has context about what it's receiving.