Implement In-Memory SSH Agent

Create a temporary SSH agent that holds certificates and private keys in memory during SSH sessions. This agent should automatically clean up expired keys and support multiple concurrent SSH connections without conflicts.

No files, tests, or entry points are named. Start by locating the existing SSH-session and key-handling paths, then determine how an in-memory agent should be scoped to sessions. Done means certificates and private keys remain in memory, expired keys are cleaned up automatically, and concurrent SSH connections do not conflict.

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