add codex sandbox {linux|macos} (#4782)

## Summary
- add a `codex sandbox` subcommand with macOS and Linux targets while
keeping the legacy `codex debug` aliases
- update documentation to highlight the new sandbox entrypoints and
point existing references to the new command
- clarify the core README about the linux sandbox helper alias

## Testing
- just fmt
- just fix -p codex-cli
- cargo test -p codex-cli


------
https://chatgpt.com/codex/tasks/task_i_68e2e00ca1e8832d8bff53aa0b50b49e

docs/sandbox.md

@@ -69,9 +69,13 @@ To test to see what happens when a command is run under the sandbox provided by
 
 ```
 # macOS
-codex debug seatbelt [--full-auto] [COMMAND]...
+codex sandbox macos [--full-auto] [COMMAND]...
 
 # Linux
+codex sandbox linux [--full-auto] [COMMAND]...
+
+# Legacy aliases
+codex debug seatbelt [--full-auto] [COMMAND]...
 codex debug landlock [--full-auto] [COMMAND]...
 ```