Add Ruby sandbox command with backends, spinner, and tests

[narkaTee]

Motivation

• Provide a Ruby implementation of the existing sandbox CLI to enable a modular, testable orchestration layer that mirrors the current Bash behavior.
• Encapsulate backend semantics (container, kvm, hcloud, proxy) behind a strict interface to simplify detection, selection and IDE/SSH integrations.
• Improve long-running operation UX with a reusable TTY spinner and add automated specs to verify core selection and naming logic.

Description

• Added a new CLI entrypoint ruby/bin/sandbox and a library under ruby/lib/sandbox/ implementing App, CLI, CommandRunner, Spinner, SshAlias, AiBootstrapper, ProxyCli, BackendInterface, ConnectionInfo, Name, Paths, and a Bash-backed backend adapter (backends/bash_backend.rb).
• Implemented concrete backend wrappers for container, kvm and hcloud in ruby/lib/sandbox/backends/ that delegate to the existing Bash backend scripts and expose backend_start, backend_stop, backend_enter, backend_is_running, backend_get_ssh_port, and backend_get_ip methods, and added proxy wrapper behavior and a .with_proxy flow.
• Implemented Spinner with TTY vs non-TTY behavior, PTY-friendly rendering model, and thread-safe task updates for long-running phases, and implemented CommandRunner that supports Open3 and PTY execution and a safe exec wrapper.
• Integrated installation and test tasks into Rakefile by adding :sandbox install task and a :test_sandbox RSpec task and added specs for naming, backend selection, spinner behavior and backend interface under ruby/lib/sandbox/spec/.

Testing

• Added RSpec tests for Sandbox::Name, backend selection in Sandbox::App, Sandbox::Spinner and Sandbox::BackendInterface under ruby/lib/sandbox/spec/ and wired them to the rake test_sandbox task.
• Ran rake test_sandbox, which printed a warning and skipped the sandbox specs because RSpec is not installed in the environment (tests therefore did not execute); the test task is functional when RSpec is available.
• Verified that installation task rake sandbox is wired into Rakefile so the Ruby sandbox binary is installed via install -m 755 ruby/bin/sandbox "$HOME/bin/sandbox" when run.

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Codex Task