[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/session/session.rs

@@ -439,7 +439,7 @@ async fn warm_plugins_and_skills_for_session_init(
     config: Arc<Config>,
     plugins_manager: Arc<PluginsManager>,
     skills_manager: Arc<SkillsManager>,
-    turn_environments: TurnEnvironmentSnapshot,
+    turn_environments: &TurnEnvironmentSnapshot,
 ) -> Vec<SkillError> {
     let fs = turn_environments.primary_filesystem();
     let plugins_input = config.plugins_config_input();
@@ -829,7 +829,7 @@ impl Session {
                 Arc::clone(&config),
                 Arc::clone(&plugins_manager),
                 Arc::clone(&skills_manager),
-                resolved_environments,
+                &resolved_environments,
             )
             .instrument(info_span!(
                 "session_init.plugin_skill_warmup",
@@ -913,8 +913,12 @@ impl Session {
                     (None, None)
                 };
 
-            let hooks =
-                build_hooks_for_config(&config, plugins_manager.as_ref(), &default_shell).await;
+            let hooks = build_hooks_for_config(
+                &config,
+                plugins_manager.as_ref(),
+                resolved_environments.single_local_environment(),
+            )
+            .await;
             for warning in hooks.startup_warnings() {
                 post_session_configured_events.push(Event {
                     id: INITIAL_SUBMIT_ID.to_owned(),