[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/src/tasks/user_shell.rs

@@ -29,6 +29,7 @@ use crate::turn_timing::now_unix_timestamp_ms;
 use crate::user_shell_command::user_shell_command_record_item;
 use codex_protocol::exec_output::ExecToolCallOutput;
 use codex_protocol::exec_output::StreamOutput;
+use codex_protocol::protocol::ErrorEvent;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::ExecCommandBeginEvent;
 use codex_protocol::protocol::ExecCommandEndEvent;
@@ -124,18 +125,26 @@ pub(crate) async fn execute_user_shell_command(
         session.send_event(turn_context.as_ref(), event).await;
     }
 
-    // Execute the user's script under their default shell when known; this
+    let Some((turn_environment, environment_shell)) = turn_context
+        .environments
+        .local()
+        .and_then(|environment| environment.shell.as_ref().map(|shell| (environment, shell)))
+    else {
+        send_user_shell_error(
+            &session,
+            turn_context.as_ref(),
+            "shell is unavailable in this session",
+        )
+        .await;
+        return;
+    };
+
+    // Execute the user's script under the environment's shell; this
     // allows commands that use shell features (pipes, &&, redirects, etc.).
     // We do not source rc files or otherwise reformat the script.
     let use_login_shell = true;
-    let session_shell = session.user_shell();
-    let environment = turn_context.environments.single_local_environment();
-    let shell = environment
-        .and_then(|environment| environment.shell.as_ref())
-        .unwrap_or(session_shell.as_ref());
-    let shell_snapshot_location =
-        environment.and_then(|environment| environment.shell_snapshot(environment.cwd()));
-    let display_command = shell.derive_exec_args(&command, use_login_shell);
+    let display_command = environment_shell.derive_exec_args(&command, use_login_shell);
+    let shell_snapshot_location = turn_environment.shell_snapshot(turn_environment.cwd());
     let mut exec_env_map = create_env(
         &turn_context.shell_environment_policy,
         Some(session.thread_id),
@@ -145,7 +154,7 @@ pub(crate) async fn execute_user_shell_command(
     }
     let exec_command = prepare_user_shell_exec_command(
         &display_command,
-        shell,
+        environment_shell,
         shell_snapshot_location.as_ref(),
         &turn_context.shell_environment_policy.r#set,
         &mut exec_env_map,
@@ -153,10 +162,7 @@ pub(crate) async fn execute_user_shell_command(
 
     let call_id = Uuid::new_v4().to_string();
     let raw_command = command;
-    #[allow(deprecated)]
-    let cwd = environment
-        .map(|environment| environment.cwd().clone())
-        .unwrap_or_else(|| turn_context.cwd.clone());
+    let cwd = turn_environment.cwd().clone();
 
     let parsed_cmd = parse_command(&display_command);
     session
@@ -341,9 +347,21 @@ pub(crate) async fn execute_user_shell_command(
     }
 }
 
+async fn send_user_shell_error(session: &Session, turn_context: &TurnContext, message: &str) {
+    session
+        .send_event(
+            turn_context,
+            EventMsg::Error(ErrorEvent {
+                message: message.to_string(),
+                codex_error_info: None,
+            }),
+        )
+        .await;
+}
+
 fn prepare_user_shell_exec_command(
     display_command: &[String],
-    session_shell: &Shell,
+    shell: &Shell,
     shell_snapshot: Option<&AbsolutePathBuf>,
     shell_environment_set: &HashMap<String, String>,
     exec_env_map: &mut HashMap<String, String>,
@@ -352,7 +370,7 @@ fn prepare_user_shell_exec_command(
     {
         prepare_user_shell_exec_command_with_path_prepend(
             display_command,
-            session_shell,
+            shell,
             shell_snapshot,
             shell_environment_set,
             exec_env_map,
@@ -364,7 +382,7 @@ fn prepare_user_shell_exec_command(
     {
         maybe_wrap_shell_lc_with_snapshot(
             display_command,
-            session_shell,
+            shell,
             shell_snapshot,
             shell_environment_set,
             exec_env_map,
@@ -384,7 +402,7 @@ fn prepare_user_shell_exec_command(
 #[cfg(unix)]
 fn prepare_user_shell_exec_command_with_path_prepend(
     display_command: &[String],
-    session_shell: &Shell,
+    shell: &Shell,
     shell_snapshot: Option<&AbsolutePathBuf>,
     shell_environment_set: &HashMap<String, String>,
     exec_env_map: &mut HashMap<String, String>,
@@ -395,7 +413,7 @@ fn prepare_user_shell_exec_command_with_path_prepend(
     prepend_runtime_path(exec_env_map, &mut runtime_path_prepends);
     maybe_wrap_shell_lc_with_snapshot(
         display_command,
-        session_shell,
+        shell,
         shell_snapshot,
         &explicit_env_overrides,
         exec_env_map,