fix(cron): warn when isolated job won't deliver output

When creating an isolated cron job with --message but without --deliver
or --to, the job runs but the agent output goes nowhere. This is a
common mistake that's hard to debug.

Add a warning at job creation time so users know they need to add
--deliver --channel <channel> --to <dest> for proactive message delivery.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/cli/cron-cli/register.cron-add.ts

@@ -177,6 +177,19 @@ export function registerCronAddCommand(cron: Command) {
             throw new Error("Isolated jobs require --message (agentTurn).");
           }
 
+          // Warn if isolated job won't deliver output anywhere
+          if (
+            sessionTarget === "isolated" &&
+            payload.kind === "agentTurn" &&
+            !payload.deliver &&
+            !payload.to
+          ) {
+            defaultRuntime.error(
+              "warning: isolated job has --message but no --deliver or --to; agent output will not be sent.\n" +
+                "Add --deliver --channel <channel> --to <dest> if you want proactive message delivery.",
+            );
+          }
+
           const isolation =
             sessionTarget === "isolated"
               ? {