[codex] reduce module visibility (#16978)

## Summary
- reduce public module visibility across Rust crates, preferring private
or crate-private modules with explicit crate-root public exports
- update external call sites and tests to use the intended public crate
APIs instead of reaching through module trees
- add the module visibility guideline to AGENTS.md

## Validation
- `cargo check --workspace --all-targets --message-format=short` passed
before the final fix/format pass
- `just fix` completed successfully
- `just fmt` completed successfully
- `git diff --check` passed

codex-rs/utils/cli/src/lib.rs

@@ -1,8 +1,9 @@
 mod approval_mode_cli_arg;
 mod config_override;
-pub mod format_env_display;
+pub(crate) mod format_env_display;
 mod sandbox_mode_cli_arg;
 
 pub use approval_mode_cli_arg::ApprovalModeCliArg;
 pub use config_override::CliConfigOverrides;
+pub use format_env_display::format_env_display;
 pub use sandbox_mode_cli_arg::SandboxModeCliArg;