feat: add CpToInstance and CpFromInstance functions

[rgarcia]

Summary

Add manually-maintained cp functionality to the Go SDK for copying files to/from running VM instances.

Usage

import "github.com/onkernel/hypeman-go/lib"

// Copy file to instance
err := lib.CpToInstance(ctx, client, instanceID, localPath, guestPath, lib.CpOptions{
    Recursive: true,
    Archive:   true,  // preserve UID/GID
})

// Copy from instance
err := lib.CpFromInstance(ctx, client, instanceID, guestPath, localPath, lib.CpOptions{
    FollowLinks: true,
})

Features

• CpToInstance: Copy local file/directory to instance
• CpFromInstance: Copy file/directory from instance to local
• WebSocket-based streaming with JSON control messages
• Supports files and recursive directory operations
• Handles symlinks, permissions, and modification times
• Chunked binary data transfer for efficient streaming

Implementation

Located in lib/cp.go following the SDK's convention for custom extensions that won't conflict with auto-generated code.

Dependencies

Requires corresponding API server changes: onkernel/hypeman#45

---

Note

Adds high-level file transfer and a new filesystem stat API.

• New lib package: CpToInstance/CpFromInstance stream files/dirs over WebSocket with progress callbacks, archive UID/GID preservation, link following, cycle detection, and path/symlink sanitization; mockable via WsConn/WsDialer (lib/ws_connector.go)
• New API: GET /instances/{id}/stat via client.Instances.Stat(...) with InstanceStatParams and PathInfo response
• Model updates: InstanceNewParams gains devices field
• Tests added for cp flows and Instances.Stat
• Deps: add github.com/gorilla/websocket and github.com/stretchr/testify; docs (api.md) and stats updated

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