feat: experimental env var: CODEX_SANDBOX_NETWORK_DISABLED

Previous to this change:

```
$ cargo run --bin codex -- debug seatbelt --full-auto -- cargo test
---- keeps_previous_response_id_between_tasks stdout ----

thread 'keeps_previous_response_id_between_tasks' panicked at /Users/mbolin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wiremock-0.6.3/src/mock_server/builder.rs:107:46:
Failed to bind an OS port for a mock server.: Os { code: 1, kind: PermissionDenied, message: "Operation not permitted" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    keeps_previous_response_id_between_tasks

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `-p codex-core --test previous_response_id`
```

codex-rs/cli/src/landlock.rs

@@ -3,10 +3,11 @@
 //! On Linux the command is executed inside a Landlock + seccomp sandbox by
 //! calling the low-level `exec_linux` helper from `codex_core::linux`.
 
+use codex_core::exec::StdioPolicy;
+use codex_core::linux::spawn_command_under_landlock;
 use codex_core::protocol::SandboxPolicy;
 use std::os::unix::process::ExitStatusExt;
 use std::process;
-use std::process::Command;
 use std::process::ExitStatus;
 
 /// Execute `command` in a Linux sandbox (Landlock + seccomp) the way Codex
@@ -19,8 +20,10 @@ pub fn run_landlock(command: Vec<String>, sandbox_policy: SandboxPolicy) -> anyh
     // Spawn a new thread and apply the sandbox policies there.
     let handle = std::thread::spawn(move || -> anyhow::Result<ExitStatus> {
         let cwd = std::env::current_dir()?;
-        codex_core::linux::apply_sandbox_policy_to_current_thread(sandbox_policy, &cwd)?;
-        let status = Command::new(&command[0]).args(&command[1..]).status()?;
+        let mut child =
+            spawn_command_under_landlock(command, &sandbox_policy, cwd, StdioPolicy::Inherit)
+                .await?;
+        let status = child.wait().await?;
         Ok(status)
     });
     let status = handle

codex-rs/cli/src/main.rs

@@ -74,6 +74,7 @@ async fn main() -> anyhow::Result<()> {
             proto::run_main(proto_cli).await?;
         }
         Some(Subcommand::Debug(debug_args)) => match debug_args.cmd {
+            #[cfg(target_os = "macos")]
             DebugCommand::Seatbelt(SeatbeltCommand {
                 command,
                 sandbox,
@@ -82,6 +83,10 @@ async fn main() -> anyhow::Result<()> {
                 let sandbox_policy = create_sandbox_policy(full_auto, sandbox);
                 seatbelt::run_seatbelt(command, sandbox_policy).await?;
             }
+            #[cfg(not(target_os = "macos"))]
+            DebugCommand::Seatbelt(_) => {
+                anyhow::bail!("Seatbelt is only supported on macOS.");
+            }
             #[cfg(target_os = "linux")]
             DebugCommand::Landlock(LandlockCommand {
                 command,

codex-rs/cli/src/seatbelt.rs

@@ -1,18 +1,24 @@
-use codex_core::exec::create_seatbelt_command;
+use codex_core::exec::StdioPolicy;
+use codex_core::exec::spawn_command_under_seatbelt;
 use codex_core::protocol::SandboxPolicy;
+use std::os::unix::process::ExitStatusExt;
+use std::process;
 
 pub async fn run_seatbelt(
     command: Vec<String>,
     sandbox_policy: SandboxPolicy,
 ) -> anyhow::Result<()> {
     let cwd = std::env::current_dir().expect("failed to get cwd");
-    let seatbelt_command = create_seatbelt_command(command, &sandbox_policy, &cwd);
-    let status = tokio::process::Command::new(seatbelt_command[0].clone())
-        .args(&seatbelt_command[1..])
-        .spawn()
-        .map_err(|e| anyhow::anyhow!("Failed to spawn command: {}", e))?
-        .wait()
-        .await
-        .map_err(|e| anyhow::anyhow!("Failed to wait for command: {}", e))?;
-    std::process::exit(status.code().unwrap_or(1));
+    let mut child =
+        spawn_command_under_seatbelt(command, &sandbox_policy, cwd, StdioPolicy::Inherit).await?;
+    let status = child.wait().await?;
+
+    // Use ExitStatus to derive the exit code.
+    if let Some(code) = status.code() {
+        process::exit(code);
+    } else if let Some(signal) = status.signal() {
+        process::exit(128 + signal);
+    } else {
+        process::exit(1);
+    }
 }