chore: refactor exec.rs: create separate seatbelt.rs and spawn.rs files (#1762)

At 550 lines, `exec.rs` was a bit large. In particular, I found it hard
to locate the Seatbelt-related code quickly without a file with
`seatbelt` in the name, so this refactors things so:

- `spawn_command_under_seatbelt()` and dependent code moves to a new
`seatbelt.rs` file
- `spawn_child_async()` and dependent code moves to a new `spawn.rs`
file

codex-rs/core/src/lib.rs

@@ -39,7 +39,9 @@ mod project_doc;
 pub mod protocol;
 mod rollout;
 mod safety;
+pub mod seatbelt;
 pub mod shell;
+pub mod spawn;
 mod user_notification;
 pub mod util;