Introduce RemoteTransport interface (Run, CopyFrom, CopyTo) with
SSHTransport implementation using ssh/scp binaries. AgentConfig gains
a Transport field with lazy initialization from M3 credentials.
All internal callers (DiscoverCheckpoints, processMLXNative,
processWithConversion) now use cfg.transport() instead of global
SSHCommand/SCPFrom. Old functions preserved as deprecated wrappers.
Enables mock injection for testing agent loop without real SSH.
Co-Authored-By: Virgil <virgil@lethean.io>