Add cp support to container machine

[kavix]

Description

Closes #2141

This PR adds file copy support (copy / cp) to container machine, allowing users to copy files and directories between the host and a container machine by machine name (e.g. container machine cp ./README.md dev:/root/ or container machine cp dev:/root/README.md ./README.md).

Previously, copying files into container machines with --home-mount none required manually inspecting the underlying container ID and invoking container cp.

Changes

• Added MachineCopy command: Implemented Application.MachineCopy in Sources/ContainerCommands/Machine/MachineCopy.swift to handle:
  • Copying from host to container machine (local:path -> machine:path).
  • Copying from container machine to host (machine:path -> local:path).
  • Resolving machine name/ID to its running containerId via MachineClient.inspect(id:).
  • Rejection of invalid paths (local-to-local and machine-to-machine).
• Subcommand registration: Added MachineCopy.self (with alias cp) under MachineCommand.configuration.subcommands.
• Testing: Added unit/validation tests in TestCLIMachineCommand.swift verifying argument validation and error handling for container machine cp.

Checklist

• Code follows the project's style guidelines.
• Added unit / integration tests covering the new behavior.
• Verified CLI help output and error handling.