fix(cron): correct false high-frequency warning for daily cron jobs

The frequency check compared two consecutive calls to
next_run_for_schedule with now and now+1s, which returned the same
next occurrence for daily schedules — making the interval appear as
0 minutes. Compare two consecutive occurrences instead.

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

src/cron/scheduler.rs

@@ -218,11 +218,14 @@ fn warn_if_high_frequency_agent_job(job: &CronJob) {
         Schedule::Every { every_ms } => *every_ms < 5 * 60 * 1000,
         Schedule::Cron { .. } => {
             let now = Utc::now();
-            match (
+            match next_run_for_schedule(&job.schedule, now) {
-                next_run_for_schedule(&job.schedule, now),
+                Ok(first) => {
-                next_run_for_schedule(&job.schedule, now + chrono::Duration::seconds(1)),
+                    // Get the occurrence *after* the first one to measure the actual interval.
-            ) {
+                    match next_run_for_schedule(&job.schedule, first) {
-                (Ok(a), Ok(b)) => (b - a).num_minutes() < 5,
+                        Ok(second) => (second - first).num_minutes() < 5,
+                        _ => false,
+                    }
+                }
                 _ => false,
             }
         }