app-server: support -c config overrides

[bolinfest]

Why

The standalone codex-app-server binary already routed a CliConfigOverrides value into app-server startup, but its own clap args did not expose the shared -c/--config option. That meant codex-app-server -c key=value was rejected before the existing config override path could run, unlike the main codex CLI.

What Changed

• Flatten CliConfigOverrides into AppServerArgs in codex-rs/app-server/src/main.rs.
• Pass parsed overrides to run_main_with_transport_options instead of always using CliConfigOverrides::default().
• Add a binary parser test covering both -c and --config for the standalone app-server.

Verification

• just test -p codex-app-server app_server_accepts_cli_config_overrides

The broader just test -p codex-app-server run was also attempted. It compiled and ran 812 tests, with 796 passing, but failed in this local sandbox on unrelated sandbox-exec: sandbox_apply: Operation not permitted command-exec/turn integration paths and a skills watcher timeout.