fix(agent): use fallback workspace path for MCP initialization

Use cfg.WorkspacePath() as a fallback when defaultAgent is nil or
its Workspace is empty. This ensures MCP servers with relative
envFile paths can always resolve them correctly, even when agents
haven't been fully initialized yet.

Previously, workspacePath would be an empty string in these cases,
causing relative envFile paths to fail to resolve. Now the fallback
guarantees a valid workspace path is always provided to
LoadFromMCPConfig.

Addresses Copilot code review feedback.

pkg/agent/loop.go

@@ -140,8 +140,10 @@ func (al *AgentLoop) Run(ctx context.Context) error {
 		mcpManager := mcp.NewManager()
 		defaultAgent := al.registry.GetDefaultAgent()
 		workspacePath := ""
-		if defaultAgent != nil {
+		if defaultAgent != nil && defaultAgent.Workspace != "" {
 			workspacePath = defaultAgent.Workspace
+		} else {
+			workspacePath = al.cfg.WorkspacePath()
 		}
 
 		if err := mcpManager.LoadFromMCPConfig(ctx, al.cfg.Tools.MCP, workspacePath); err != nil {