refactor(bus,channels): promote peer and messageID from metadata to structured fields

Add bus.Peer struct and explicit Peer/MessageID fields to InboundMessage,
replacing the implicit peer_kind/peer_id/message_id metadata convention.

- Add Peer{Kind, ID} type to pkg/bus/types.go
- Extend InboundMessage with Peer and MessageID fields
- Change BaseChannel.HandleMessage signature to accept peer and messageID
- Adapt all 12 channel implementations to pass structured peer/messageID
- Simplify agent extractPeer() to read msg.Peer directly
- extractParentPeer unchanged (parent_peer still via metadata)

pkg/bus/types.go

@@ -1,11 +1,19 @@
 package bus
 
+// Peer identifies the routing peer for a message (direct, group, channel, etc.)
+type Peer struct {
+	Kind string `json:"kind"` // "direct" | "group" | "channel" | ""
+	ID   string `json:"id"`
+}
+
 type InboundMessage struct {
 	Channel    string            `json:"channel"`
 	SenderID   string            `json:"sender_id"`
 	ChatID     string            `json:"chat_id"`
 	Content    string            `json:"content"`
 	Media      []string          `json:"media,omitempty"`
+	Peer       Peer              `json:"peer"`                 // routing peer
+	MessageID  string            `json:"message_id,omitempty"` // platform message ID
 	SessionKey string            `json:"session_key"`
 	Metadata   map[string]string `json:"metadata,omitempty"`
 }