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

cmd/picoclaw/cmd_gateway.go

@@ -33,6 +33,7 @@ import (
 	"github.com/sipeed/picoclaw/pkg/health"
 	"github.com/sipeed/picoclaw/pkg/heartbeat"
 	"github.com/sipeed/picoclaw/pkg/logger"
+	"github.com/sipeed/picoclaw/pkg/media"
 	"github.com/sipeed/picoclaw/pkg/providers"
 	"github.com/sipeed/picoclaw/pkg/state"
 	"github.com/sipeed/picoclaw/pkg/tools"
@@ -123,14 +124,18 @@ func gatewayCmd() {
 		return tools.SilentResult(response)
 	})
 
-	channelManager, err := channels.NewManager(cfg, msgBus)
+	// Create media store for file lifecycle management
+	mediaStore := media.NewFileMediaStore()
+
+	channelManager, err := channels.NewManager(cfg, msgBus, mediaStore)
 	if err != nil {
 		fmt.Printf("Error creating channel manager: %v\n", err)
 		os.Exit(1)
 	}
 
-	// Inject channel manager into agent loop for command handling
+	// Inject channel manager and media store into agent loop
 	agentLoop.SetChannelManager(channelManager)
+	agentLoop.SetMediaStore(mediaStore)
 
 	var transcriber *voice.GroqTranscriber
 	groqAPIKey := cfg.Providers.Groq.APIKey