Route shell commands through exec environments

[starr-openai]

Summary

• route shell execution through the bound exec environment when experimental_exec_server_url is configured
• keep the existing local execution path when no remote exec-server is bound
• add coverage for split stdout/stderr collection from the exec-server path

Validation

• cargo test -p codex-core user_shell_cmd_ls_and_cat_in_temp_dir -- --nocapture
• cargo test -p codex-core consume_exec_server_output_collects_split_streams -- --nocapture
• cargo test -p codex-core shell_command_works -- --nocapture
• cargo test -p codex-app-server thread_shell_command_runs_as_standalone_turn_and_persists_history -- --nocapture

Remote validation

• verified on applied devbox dev-codex-fast2-0320-113526 with remote codex-exec-server
• cargo test -p codex-core consume_exec_server_output_collects_split_streams -- --nocapture
• cargo test -p codex-core shell_command_works -- --nocapture

[github-actions]

Closing this pull request because it has had no updates for more than 14 days. If you plan to continue working on it, feel free to reopen or open a new PR.