[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/environment_selection.rs

@@ -194,6 +194,12 @@ impl TurnEnvironmentSnapshot {
         self.turn_environments.first()
     }
 
+    pub(crate) fn local(&self) -> Option<&TurnEnvironment> {
+        self.turn_environments
+            .iter()
+            .find(|environment| !environment.environment.is_remote())
+    }
+
     #[cfg(test)]
     pub(crate) fn primary_environment(&self) -> Option<Arc<codex_exec_server::Environment>> {
         self.primary()