feat: add WebSocket endpoint for cp operations

Add /instances/{id}/cp WebSocket endpoint to enable file copy operations
between the local filesystem and running VM instances. Also adds
/instances/{id}/stat HTTP endpoint for querying guest path metadata.

Features:
- Copy files and directories to/from running instances
- Streaming file transfer with chunked data
- Support for symlinks and directory hierarchies
- UID/GID preservation for archive mode
- Follow symlinks option

Author: rgarcia