[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/tools/runtimes/unified_exec.rs

@@ -322,7 +322,7 @@ impl<'a> ToolRuntime<UnifiedExecRequest, UnifiedExecProcess> for UnifiedExecRunt
             attempt.sandbox,
             attempt.windows_sandbox_level,
         );
-        let command = if matches!(session_shell.shell_type, ShellType::PowerShell) {
+        let command = if matches!(req.shell_type, ShellType::PowerShell) {
             prefix_powershell_script_with_utf8(&command)
         } else {
             command