Route unified exec and filesystem tools through exec-server

Add exec-server filesystem RPCs and a core-side remote filesystem client,
then route unified-exec and filesystem-backed tools through that backend
when enabled by config. Also add Docker-backed remote exec integration
coverage for the local codex-exec CLI.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/app-server/src/fs_api.rs

@@ -34,7 +34,7 @@ pub(crate) struct FsApi {
 impl Default for FsApi {
     fn default() -> Self {
         Self {
-            file_system: Arc::new(Environment.get_filesystem()),
+            file_system: Environment::default().get_filesystem(),
         }
     }
 }