fix: pretty-print the sandbox config in the TUI/exec modes (#1376)

Now that https://github.com/openai/codex/pull/1373 simplified the
sandbox config, we can print something much simpler in the TUI (and in
`codex exec`) to summarize the sandbox config.

Before:

![Screenshot 2025-06-24 at 5 45
52 PM](https://github.com/user-attachments/assets/b7633efb-a619-43e1-9abe-7bb0be2d0ec0)

With this change:

![Screenshot 2025-06-24 at 5 46
44 PM](https://github.com/user-attachments/assets/8d099bdd-a429-4796-a08d-70931d984e4f)

For reference, my `config.toml` contains:

```
[sandbox]
mode = "workspace-write"
writable_roots = ["/tmp", "/Users/mbolin/.pyenv/shims"]
```

Fixes https://github.com/openai/codex/issues/1248

codex-rs/common/Cargo.toml

@@ -16,3 +16,4 @@ serde = { version = "1", optional = true }
 # Separate feature so that `clap` is not a mandatory dependency.
 cli = ["clap", "toml", "serde"]
 elapsed = []
+sandbox_summary = []