Integrate Arcjet Guard into Python Strands Agents

arcjet.guard.strands_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 Strands-specific.

Official Python strands / strands-agents>=1.11.0,<2 only — not JS @arcjet/guard/strands-agents/v1 (docs https://docs.arcjet.com/guards/strands-agents/), not community forks. Importing arcjet.guard.strands_agents does not load LangChain, CrewAI, or JS @arcjet/guard/strands-agents/v1. Unlike CrewAI (no extra), the arcjet[strands-agents] extra exists at the pin (strands-agents = ["strands-agents>=1.11.0,<2"] in pyproject.toml). The extra is safe (no chromadb).

Exports: guard_tool, guard_hooks, strands_agent_context. Authored @tool plus Agent(hooks=) / add_hook.