Harden exec-server unified-exec follow-up

- fall back to local when sandboxed exec cannot be modeled remotely
- use server-issued process ids for remote session continuations
- retain symlink fidelity across fs/readDirectory plumbing
- clean up exited exec-server processes after retention

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

codex-rs/core/src/exec_server_filesystem.rs

@@ -99,7 +99,7 @@ impl ExecutorFileSystem for ExecServerFileSystem {
                 file_name: entry.file_name,
                 is_directory: entry.is_directory,
                 is_file: entry.is_file,
-                is_symlink: false,
+                is_symlink: entry.is_symlink,
             })
             .collect())
     }