feat: add cp command for file copy operations

Add 'hypeman cp' command for copying files to/from running instances,
with docker cp compatible semantics:

Features:
- Copy files and directories to instances: hypeman cp file.txt instance:/path
- Copy from instances: hypeman cp instance:/path file.txt
- STDIN/STDOUT streaming with '-' argument for tar archives
- Recursive directory copying with proper tar packing/unpacking
- Symlink handling with --follow-links flag
- Archive mode (-a) to preserve UID/GID ownership
- Quiet mode (-q) to suppress progress output
- Docker cp path resolution semantics:
  - SRC_PATH/. copies contents, not the directory itself
  - Trailing slash handling for directory vs file detection

The command communicates with the API server via WebSocket,
streaming file data as binary chunks with JSON metadata headers.

Author: rgarcia

pkg/cmd/cmd.go

@@ -68,6 +68,7 @@ func init() {
 		},
 		Commands: []*cli.Command{
 			&execCmd,
+			&cpCmd,
 			&pullCmd,
 			&pushCmd,
 			&runCmd,