app-server: preserve target-native environment cwd (#28146)

## Why

app-server may run on a different OS from the selected exec-server
environment. Parsing that environment’s cwd with the Codex host’s path
rules prevents thread startup.

## What

Carry environment cwd values as `LegacyAppPathString` at the app-server
boundary and `PathUri` internally. Existing tool-call schemas and
relative-path behavior stay host-native; remaining local-only consumers
convert explicitly and leave follow-up TODOs.

The Wine integration test verifies app-server can start a thread and
complete an ordinary turn with a Windows environment cwd from Linux.

## Validation

- `bazel test //codex-rs/core/tests/remote_env_windows:smoke-test
--test_output=errors`
- focused app-server environment-selection and protocol schema tests
- scoped Clippy for `codex-core` and `codex-app-server-protocol`

codex-rs/core/src/agents_md.rs

@@ -53,11 +53,16 @@ pub(crate) async fn load_project_instructions(
     let mut loaded = LoadedAgentsMd::from_user_instructions(user_instructions);
     for turn_environment in &environments.turn_environments {
         let filesystem = turn_environment.environment.get_filesystem();
+        // TODO(anp): Migrate AGENTS.md discovery to PathUri so instructions can be loaded from
+        // environment-native foreign working directories.
+        let Ok(cwd) = turn_environment.cwd().to_abs_path() else {
+            continue;
+        };
         match read_agents_md(
             config,
             filesystem.as_ref(),
             &turn_environment.environment_id,
-            turn_environment.cwd(),
+            &cwd,
         )
         .await
         {