Close session and return to connections after Ctrl+D

[ocnc]

Summary

• fix SSH shell-exit handling so child-channel close (Ctrl+D logout) is surfaced explicitly as a transport session end
• remove the architecture leak by keeping TerminalSession transport-agnostic (no SSHTransportError inspection)
• wire session-end lifecycle in SessionManager so ended sessions are removed, returning to the connections list when no sessions remain

Architecture

• added TransportState.sessionEnded to model terminal-end semantics at the transport boundary
• SSHTransport now emits .sessionEnded when the SSH session channel closes

Validation

• swift test --package-path Packages/SpecttyTransport
• xcodebuild -project Spectty.xcodeproj -scheme Spectty -configuration Debug -destination 'generic/platform=iOS Simulator' build

Closes #7