Integrate Arcjet Guard into the Python Claude Agent SDK

arcjet.guard.claude_agent_sdk 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 Agent SDK-specific.

Official claude-agent-sdk>=0.2.127,<1 only — not the JS @anthropic-ai/claude-agent-sdk adapter (@arcjet/guard/claude-agent-sdk/v0, docs https://docs.arcjet.com/guards/claude-agent-sdk/), not Vercel AI, not community forks, and not Claude Managed Agents hosted client.beta.sessions (arcjet.guard.claude_managed_agents). Importing arcjet.guard.claude_agent_sdk does not load LangChain, CrewAI, or the JS adapter. The extra is safe (no chromadb).