Integrate Arcjet Guard into Microsoft Agent Framework for Go

github.com/arcjet/arcjet-go/agentframework wraps the application's existing arcjet.GuardClient. Shared Guard fundamentals (client, rules, labels, decisions, capture) live in ../arcjet/references/guards_go.md. Load that reference for anything that is not framework-specific.

Three surfaces, one decision rule:

• Any Go function → arcjet.GuardAction in the root module. No framework dependency. Returns *arcjet.GuardDeniedError or *arcjet.GuardUnavailableError.
• A tool.FuncTool you can name at wiring time (from functool.New, mcptool.ListTools, or agenttool.New) → agentframework.GuardTool, or GuardTools for a list. The result is still a tool.FuncTool.
• An agent whose tools the model picks, or user text to screen → agentframework.GuardMiddleware in agent.Config.Middlewares.