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/cli/src/debug_sandbox.rs

@@ -4,10 +4,10 @@ use codex_common::CliConfigOverrides;
 use codex_core::config::Config;
 use codex_core::config::ConfigOverrides;
 use codex_core::config_types::SandboxMode;
-use codex_core::exec::StdioPolicy;
 use codex_core::exec::spawn_command_under_linux_sandbox;
-use codex_core::exec::spawn_command_under_seatbelt;
 use codex_core::exec_env::create_env;
+use codex_core::seatbelt::spawn_command_under_seatbelt;
+use codex_core::spawn::StdioPolicy;
 
 use crate::LandlockCommand;
 use crate::SeatbeltCommand;