fix(agent): stop the agent and close redis on shutdown, document the single bean

[rennf93]

Follow-ups from the review of #18. GuardMiddleware.shutdown() now runs under the initialization mutex, so it cannot interleave with a first-request initialize(), and is safe to call twice. It also closes the middleware's own redisHandler, and when initialize() never ran it stops the injected agent handler directly, so a GuardAgent created with cfg.redis no longer leaks its persistence connection and daemon thread. The README now states that exactly one AgentHandlerProtocol bean is used (two fail startup unless one is @Primary) and gives the guard-agent-jvm dependency line. I added tests for context close, shutdown without initialize, double shutdown and the redis close.