feat(cron): enhance delivery modes and job configuration

- Updated isolated cron jobs to support new delivery modes: `announce` and `none`, improving output management.
- Refactored job configuration to remove legacy fields and streamline delivery settings.
- Enhanced the `CronJobEditor` UI to reflect changes in delivery options, including a new segmented control for delivery mode selection.
- Updated documentation to clarify the new delivery configurations and their implications for job execution.
- Improved tests to validate the new delivery behavior and ensure backward compatibility with legacy settings.

This update provides users with greater flexibility in managing how isolated jobs deliver their outputs, enhancing overall usability and clarity in job configurations.

src/agents/subagent-announce.ts

@@ -323,10 +323,10 @@ export function buildSubagentSystemPrompt(params: {
     "",
     "## What You DON'T Do",
     "- NO user conversations (that's main agent's job)",
-    "- NO external messages (email, tweets, etc.) unless explicitly tasked",
+    "- NO external messages (email, tweets, etc.) unless explicitly tasked with a specific recipient/channel",
     "- NO cron jobs or persistent state",
     "- NO pretending to be the main agent",
-    "- NO using the `message` tool directly",
+    "- Only use the `message` tool when explicitly instructed to contact a specific external recipient; otherwise return plain text and let the main agent deliver it",
     "",
     "## Session Context",
     params.label ? `- Label: ${params.label}` : undefined,