fix: implement source filter for /messages and /messages/new endpoints

The ?source= query parameter was documented but never implemented.
Both endpoints now properly filter by source when specified.

main.go

@@ -279,6 +279,18 @@ func handleMessages(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Filter by source if specified
+	sourceFilter := r.URL.Query().Get("source")
+	if sourceFilter != "" {
+		var filtered []UnifiedMessage
+		for _, msg := range messages {
+			if msg.Source == sourceFilter {
+				filtered = append(filtered, msg)
+			}
+		}
+		messages = filtered
+	}
+
 	json.NewEncoder(w).Encode(messages)
 }
 
@@ -294,9 +306,17 @@ func handleMessagesNew(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Filter by source if specified
+	sourceFilter := r.URL.Query().Get("source")
+
 	// Filter by orchestration state and record new messages
 	var filtered []UnifiedMessage
 	for _, msg := range messages {
+		// Skip if source filter is set and doesn't match
+		if sourceFilter != "" && msg.Source != sourceFilter {
+			continue
+		}
+
 		// Record that we've seen this message
 		source, sourceID, _ := parseMessageID(msg.ID)
 		orch.RecordSeen(msg.ID, source, sourceID, "INBOX")