[codex] Use local environment for user shell commands (#28163)

## Why

User shell commands still read the legacy turn cwd and session shell
even though execution context is now owned by selected turn
environments. App-server also defines `thread/shellCommand` as a
local-host escape hatch, so it must use an available local environment
even when a remote environment is primary.

## What changed

- Add `ResolvedTurnEnvironments::local()` to find the selected local
environment.
- Resolve the user shell command cwd and shell from that local
`TurnEnvironment`.
- Emit the standard `shell is unavailable in this session` error when no
selected local environment or resolved local shell is available.
- Add an integration test covering `/shell` without a local environment.

## Test plan

- `just test -p codex-core
user_shell_command_without_local_environment_emits_error`

codex-rs/core/tests/suite/user_shell_cmd.rs

@@ -21,6 +21,7 @@ use core_test_support::responses::mount_sse_once;
 use core_test_support::responses::sse;
 use core_test_support::responses::start_mock_server;
 use core_test_support::skip_if_no_network;
+use core_test_support::submit_thread_settings;
 use core_test_support::test_codex::local_selections;
 use core_test_support::test_codex::test_codex;
 use core_test_support::test_codex::turn_permission_fields;
@@ -99,6 +100,40 @@ async fn user_shell_cmd_ls_and_cat_in_temp_dir() {
     assert_eq!(stdout, contents);
 }
 
+#[tokio::test]
+async fn user_shell_command_without_local_environment_emits_error() -> anyhow::Result<()> {
+    let server = start_mock_server().await;
+    let mut builder = test_codex();
+    let test = builder.build(&server).await?;
+    submit_thread_settings(
+        &test.codex,
+        codex_protocol::protocol::ThreadSettingsOverrides {
+            environments: Some(codex_protocol::protocol::TurnEnvironmentSelections::new(
+                test.config.cwd.clone(),
+                vec![],
+            )),
+            ..Default::default()
+        },
+    )
+    .await?;
+
+    test.codex
+        .submit(Op::RunUserShellCommand {
+            command: "echo shell".to_string(),
+        })
+        .await?;
+
+    let EventMsg::Error(error) =
+        wait_for_event(&test.codex, |event| matches!(event, EventMsg::Error(_))).await
+    else {
+        unreachable!()
+    };
+    assert_eq!(error.message, "shell is unavailable in this session");
+    assert_eq!(error.codex_error_info, None);
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn user_shell_cmd_can_be_interrupted() {
     // Set up isolated config and conversation.