refactor(heartbeat): simplify service with single handler and direct bus usage

- Remove redundant ChannelSender interface, use *bus.MessageBus directly
- Consolidate two handlers (onHeartbeat, onHeartbeatWithTools) into one
- Move HEARTBEAT.md and heartbeat.log to workspace root
- Simplify NewHeartbeatService signature (remove handler param)
- Add SetBus and SetHandler methods for dependency injection

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

cmd/picoclaw/main.go

@@ -654,10 +654,20 @@ func gatewayCmd() {
 
 	heartbeatService := heartbeat.NewHeartbeatService(
 		cfg.WorkspacePath(),
-		nil,
-		30*60,
+		30,
 		cfg.Heartbeat.Enabled,
 	)
+	heartbeatService.SetBus(msgBus)
+	heartbeatService.SetHandler(func(prompt string) *tools.ToolResult {
+		response, err := agentLoop.ProcessDirect(context.Background(), prompt, "heartbeat")
+		if err != nil {
+			return tools.ErrorResult(fmt.Sprintf("Heartbeat error: %v", err))
+		}
+		if response == "HEARTBEAT_OK" {
+			return tools.SilentResult("Heartbeat OK")
+		}
+		return tools.UserResult(response)
+	})
 
 	channelManager, err := channels.NewManager(cfg, msgBus)
 	if err != nil {