refactor(media): add MediaStore for unified media file lifecycle management

Channels previously deleted downloaded media files via defer os.Remove,
racing with the async Agent consumer. Introduce MediaStore to decouple
file ownership: channels register files on download, Agent releases them
after processing via ReleaseAll(scope).

- New pkg/media with MediaStore interface + FileMediaStore implementation
- InboundMessage gains MediaScope field for lifecycle tracking
- BaseChannel gains SetMediaStore/GetMediaStore + BuildMediaScope helper
- Manager injects MediaStore into channels; AgentLoop releases on completion
- Telegram, Discord, Slack, OneBot, LINE channels migrated from defer
  os.Remove to store.Store() with media:// refs

pkg/bus/types.go

@@ -12,8 +12,9 @@ type InboundMessage struct {
 	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
+	Peer       Peer              `json:"peer"`                  // routing peer
+	MessageID  string            `json:"message_id,omitempty"`  // platform message ID
+	MediaScope string            `json:"media_scope,omitempty"` // media lifecycle scope
 	SessionKey string            `json:"session_key"`
 	Metadata   map[string]string `json:"metadata,omitempty"`
 }