Integrate Arcjet Guard into Python Claude Managed Agents

arcjet.guard.claude_managed_agents wraps the agent's existing Arcjet client. It never talks to the Arcjet API itself. Shared Guard fundamentals (client, rules, labels, decisions, capture, registration) live in ../arcjet/references/guards_python.md. Load that reference for anything that is not Claude Managed Agents-specific.

Official anthropic>=0.92.0,<2 only — not claude-agent-sdk, not arcjet.guard.claude_agent_sdk, not the JS @anthropic-ai/claude-agent-sdk adapter, and not JS Claude Managed Agents (@arcjet/guard/claude-managed-agents/v0, docs https://docs.arcjet.com/guards/claude-managed-agents/). Importing arcjet.guard.claude_managed_agents does not load Claude Agent SDK, LangChain, or CrewAI. The extra is safe (no chromadb).