This PR is a straight refactor so that creating the `Child` process for
an `shell` tool call and consuming its output can be separate concerns.
For the actual tool call, we will always apply
`consume_truncated_output()`, but for the top-level debug commands in
the CLI (e.g., `debug seatbelt` and `debug landlock`), we only want to
use the `spawn_child()` part of `exec()`.
We want the subcommands to match the `shell` tool call usage as
faithfully as possible. This becomes more important when we introduce a
new parameter to `spawn_child()` in
https://github.com/openai/codex/pull/879.
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* #879
* __->__ #878
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