fix(cron): clarify agent-turn can use all tools in description

The model was confused about payload capabilities. Explicitly state
that agent-turn spawns an isolated agent with ALL tools (exec, write,
web_fetch, etc.) and add usage guidance for choosing between
system-event and agent-turn.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/agent/tools/cron/cron-tool.ts

@@ -157,11 +157,14 @@ SCHEDULE TYPES (schedule.kind):
   { "kind": "cron", "expr": "0 9 * * *", "tz": "Asia/Shanghai" }
 
 PAYLOAD TYPES (payload.kind):
-- "system-event": Injects text as system event into session
+- "system-event": Injects text into main session (like a reminder, triggers main agent to respond)
   { "kind": "system-event", "text": "<message>" }
-- "agent-turn": Runs agent with message (isolated sessions only)
+- "agent-turn": Spawns an isolated agent that can use ALL tools (exec, write, web_fetch, etc.) to autonomously complete a task
   { "kind": "agent-turn", "message": "<prompt>", "timeoutSeconds": 300 }
 
+USE "agent-turn" when the job needs to perform actions (run commands, write files, fetch data, etc.).
+USE "system-event" when the job only needs to remind/notify the user in the current chat.
+
 CRITICAL CONSTRAINTS:
 - sessionTarget="main" REQUIRES payload.kind="system-event"
 - sessionTarget="isolated" REQUIRES payload.kind="agent-turn"