fix: add missing platforms to cron/send_message delivery maps and tool schema

Matrix, Mattermost, Home Assistant, and DingTalk were missing from the
platform_map in both cron/scheduler.py and tools/send_message_tool.py,
causing delivery to those platforms to silently fail.

Also updates the cronjob tool schema description to list all available
delivery targets so the model knows its options.

cron/scheduler.py

@@ -137,6 +137,9 @@ def _deliver_result(job: dict, content: str) -> None:
         "whatsapp": Platform.WHATSAPP,
         "signal": Platform.SIGNAL,
         "matrix": Platform.MATRIX,
+        "mattermost": Platform.MATTERMOST,
+        "homeassistant": Platform.HOMEASSISTANT,
+        "dingtalk": Platform.DINGTALK,
         "email": Platform.EMAIL,
         "sms": Platform.SMS,
     }