feat(monitor): channels working — inbox uses {data:[...]} + debug events

Fixed inbox API response parsing: {data:[...]} not {messages:[...]}.
Added debugChannel helper for real-time debug events via channels.
MONITOR_INTERVAL env override for faster iteration.

Channels confirmed working:
- agent.complete: workspace status changes
- inbox.message: unread message notifications
- monitor.debug: debug trace events

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/monitor/monitor.go

@@ -322,9 +322,12 @@ func (m *Subsystem) checkInbox() string {
 		} `json:"data"`
 	}
 	if json.NewDecoder(httpResp.Body).Decode(&resp) != nil {
+		m.debugChannel("checkInbox: failed to decode response")
 		return ""
 	}
 
+	m.debugChannel(fmt.Sprintf("checkInbox: got %d messages", len(resp.Data)))
+
 	unread := 0
 	senders := make(map[string]int)
 	latestSubject := ""