Fix scheduler cleanup edge cases

agent/internal/config/config.go

@@ -217,7 +217,7 @@ func (c *Config) Validate() error {
 			return fmt.Errorf("check %q: timeout must be > 0", ch.ID)
 		}
 		if ch.Interval.Duration < 0 {
-			return fmt.Errorf("check %q: interval must be > 0", ch.ID)
+			return fmt.Errorf("check %q: interval must not be negative", ch.ID)
 		}
 		ch.Cron = strings.TrimSpace(ch.Cron)
 		hasInterval := ch.Interval.Duration > 0

agent/internal/config/config_test.go

@@ -123,6 +123,29 @@ func TestValidateCheckScheduleChoice(t *testing.T) {
 	}
 }
 
+func TestValidateNegativeIntervalMessage(t *testing.T) {
+	_, err := Load(writeTmp(t, `
+agent_id = "host-1"
+state_dir = "/tmp/x"
+
+[server]
+enabled = true
+url = "http://localhost"
+
+[[checks]]
+id = "c1"
+command = "true"
+interval = "-1s"
+timeout = "5s"
+`))
+	if err == nil {
+		t.Fatal("expected negative interval error")
+	}
+	if !strings.Contains(err.Error(), "interval must not be negative") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
 func TestLoadResourceLimits(t *testing.T) {
 	c, err := Load(writeTmp(t, `
 	agent_id = "host-1"