[codex] preserve explicit environment cwd (#27995)

## Why

`TurnEnvironmentSelections::new` rewrote the primary environment's
explicit `cwd` to the legacy fallback cwd. For a remote-first selection,
this could replace the remote working directory with a local fallback
path and made the legacy cwd overlay authoritative over
environment-owned state.

## What changed

- Preserve every explicit environment cwd when constructing turn
environment selections.
- Keep `cwd`-only app-server updates compatible by rebuilding the
default environment selections at the requested cwd.
- Cover both explicit primary cwd preservation and cwd-only updates
reaching the model-visible execution environment.

## Testing

- `just test -p codex-core
session_update_settings_does_not_rewrite_sticky_environment_cwds`
- `just test -p codex-core
environment_settings_preserve_explicit_primary_cwd`
- `just test -p codex-app-server
thread_settings_update_cwd_retargets_default_environment`

codex-rs/protocol/src/protocol.rs

@@ -106,7 +106,6 @@ pub const COLLABORATION_MODE_CLOSE_TAG: &str = "</collaboration_mode>";
 pub const REALTIME_CONVERSATION_OPEN_TAG: &str = "<realtime_conversation>";
 pub const REALTIME_CONVERSATION_CLOSE_TAG: &str = "</realtime_conversation>";
 pub const USER_MESSAGE_BEGIN: &str = "## My request for Codex:";
-const LOCAL_ENVIRONMENT_ID: &str = "local";
 
 // TODO(anp): Replace `TurnEnvironmentSelection` with `PathUri` once path URIs carry environment
 // identifiers.
@@ -127,22 +126,9 @@ impl TurnEnvironmentSelections {
         legacy_fallback_cwd: AbsolutePathBuf,
         environments: Vec<TurnEnvironmentSelection>,
     ) -> Self {
-        let mut settings = Self {
+        Self {
             legacy_fallback_cwd,
             environments,
-        };
-        settings.sync_primary_environment_cwd();
-        settings
-    }
-
-    fn sync_primary_environment_cwd(&mut self) {
-        let legacy_fallback_cwd = PathUri::from_abs_path(&self.legacy_fallback_cwd);
-        // Keep remote environments' native cwd instead of replacing it with the local fallback.
-        if let Some(turn_environment) = self.environments.first_mut()
-            && turn_environment.environment_id == LOCAL_ENVIRONMENT_ID
-            && turn_environment.cwd != legacy_fallback_cwd
-        {
-            turn_environment.cwd = legacy_fallback_cwd;
         }
     }
 }