[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/codex-api/tests/models_integration.rs

@@ -1,7 +1,7 @@
 use codex_api::AuthProvider;
 use codex_api::ModelsClient;
-use codex_api::provider::Provider;
-use codex_api::provider::RetryConfig;
+use codex_api::Provider;
+use codex_api::RetryConfig;
 use codex_client::ReqwestTransport;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::openai_models::ConfigShellToolType;