merge: resolve conflict with upstream main

Merge upstream changes (HeartbeatConfig addition) alongside LINEConfig.

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

pkg/channels/manager.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/sipeed/picoclaw/pkg/bus"
 	"github.com/sipeed/picoclaw/pkg/config"
+	"github.com/sipeed/picoclaw/pkg/constants"
 	"github.com/sipeed/picoclaw/pkg/logger"
 )
 
@@ -242,6 +243,11 @@ func (m *Manager) dispatchOutbound(ctx context.Context) {
 				continue
 			}
 
+			// Silently skip internal channels
+			if constants.IsInternalChannel(msg.Channel) {
+				continue
+			}
+
 			m.mu.RLock()
 			channel, exists := m.channels[msg.Channel]
 			m.mu.RUnlock()