feat: add devbox code command for IDE Remote SSH

[ethannortharc]

Summary

• Adds devbox code command to open VS Code, Cursor, Windsurf (or any Remote SSH editor) directly into a sandbox VM
• Automatically configures ~/.ssh/config with managed markers for clean updates
• Supports both Lima (macOS) and Incus (Linux) runtimes

Usage

devbox code              # VS Code into current sandbox
devbox code myapp        # Named sandbox
devbox code --editor cursor   # Use Cursor
devbox code --path /workspace/src  # Open specific path

How it works

• Lima: Extracts SSH config via limactl show-ssh --format config, writes to ~/.ssh/config
• Incus: Gets VM IP from incus list --format json, generates SSH config entry
• Both launch editor with --remote=ssh-remote+devbox-<name>/workspace

Test plan

• devbox code opens VS Code into a running Lima sandbox (macOS)
• devbox code --editor cursor works with Cursor
• devbox code myapp resolves named sandbox correctly
• SSH config is written with # devbox-start:/end: markers
• Running devbox code twice updates (not duplicates) the SSH config block
• Error message shown when sandbox not found or editor not installed

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