cli: infer host sandbox backend (#24102)

## Why

`codex sandbox` previously required an OS subcommand like `linux`,
`macos`, or `windows`, even though the command can only run the sandbox
backend available on the current host. That made the CLI imply a
cross-OS choice that does not exist.

## What changed

- Collapse `codex sandbox <os>` into `codex sandbox [COMMAND]...` by
wiring the `sandbox` parser directly to the host-specific backend args
with `cfg`.
- Keep the existing backend runners for Seatbelt, Linux sandbox, and
Windows restricted token.
- Rename the public Windows debug sandbox runner to
`run_command_under_windows_sandbox` for clarity.
- Update the Rust sandbox docs and related README references to describe
host OS selection and avoid pointing readers at legacy `sandbox_mode`
config.

## Arg0 compatibility

The `codex-linux-sandbox` helper path is still handled before normal CLI
parsing. `arg0_dispatch()` checks whether the executable basename is
`codex-linux-sandbox` and directly calls
`codex_linux_sandbox::run_main()`, so removing the `sandbox linux`
parser branch does not affect the arg0 helper flow.

## Verification

- `cargo test -p codex-cli`
- `cargo test -p codex-arg0`
- `just fix -p codex-cli`

codex-rs/cli/src/lib.rs

@@ -9,7 +9,7 @@ use std::path::PathBuf;
 
 pub use debug_sandbox::run_command_under_landlock;
 pub use debug_sandbox::run_command_under_seatbelt;
-pub use debug_sandbox::run_command_under_windows;
+pub use debug_sandbox::run_command_under_windows_sandbox;
 pub use login::read_access_token_from_stdin;
 pub use login::read_api_key_from_stdin;
 pub use login::run_login_status;
@@ -20,8 +20,8 @@ pub use login::run_login_with_device_code;
 pub use login::run_login_with_device_code_fallback_to_browser;
 pub use login::run_logout;
 
-// TODO: Deduplicate these shared sandbox options if we remove the explicit
-// `codex sandbox <os>` platform subcommands.
+// These command structs share common sandbox options, but remain separate
+// because each host backend has a slightly different option surface.
 #[derive(Debug, Parser)]
 pub struct SeatbeltCommand {
     /// Named permissions profile to apply from the active configuration stack.