feat: add approval handler for VM vsock communication (#495)

* feat: add approval handler for VM vsock communication

- Create ApprovalHandler that implements VsockMessageHandler
- Allow guest VMs to request approval for red actions via vsock
- Support all change types: file_create, file_edit, file_delete, command_exec, email_send, external_call, asset_transfer
- Add test mode for auto-approval in testing scenarios
- Integrate with existing ApprovalManager and TUI system

Closes #493

* fix: gate approval_handler module with cfg(unix) for Windows compatibility

The approval_handler module imports VsockMessageHandler from the vsock
module, which is only available on Unix systems. This fix gates the
approval_handler module with #[cfg(unix)] to prevent Windows build failures.

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Co-authored-by: Auto Merge Bot <auto-merge@ironclaw.app>

Author: anchapin