- Add keep_alive_interval and keep_alive_max_count to SshSecurityConfig
- Enterprise default: 15s interval, 3 max failures
- Development default: 30s interval, 5 max failures
- Track last_activity timestamp in service loop
- Send keepalive@openssh.com channel request when idle
- Disconnect after max keepalive failures
- Add build_keepalive_request() and get_first_session_channel()
- Prevents connection timeout on idle SSH sessions
All 229 tests pass.