Implement ConnectOptions struct and ClientSession orchestration with graceful shutdown

Adds client/connect.rs with ConnectOptions (programmatic API per ADR-011),
ClientSession::new() for SSH session establishment, ClientSession::run()
for SOCKS5 + port forwards + shutdown, and graceful shutdown via
SIGTERM/SIGINT with SSH disconnect and 2s drain timeout.

crates/wraith-core/src/client/mod.rs

@@ -1,5 +1,7 @@
 pub mod channel_manager;
+pub mod connect;
 pub mod forward;
 
 pub use channel_manager::{ChannelManager, ForwardRequest};
+pub use connect::{ClientSession, ConnectError, ConnectOptions, TransportMode};
 pub use forward::{LocalForwarder, PortForwardSpec, PortForwardSpecKind, RemoteForwarder};