[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/core/src/client.rs

@@ -31,12 +31,15 @@ use std::sync::atomic::AtomicBool;
 use std::sync::atomic::AtomicU64;
 use std::sync::atomic::Ordering;
 
+use codex_api::ApiError;
 use codex_api::CompactClient as ApiCompactClient;
 use codex_api::CompactionInput as ApiCompactionInput;
+use codex_api::Compression;
 use codex_api::MemoriesClient as ApiMemoriesClient;
 use codex_api::MemorySummarizeInput as ApiMemorySummarizeInput;
 use codex_api::MemorySummarizeOutput as ApiMemorySummarizeOutput;
 use codex_api::RawMemory as ApiRawMemory;
+use codex_api::Reasoning;
 use codex_api::RequestTelemetry;
 use codex_api::ReqwestTransport;
 use codex_api::ResponseCreateWsRequest;
@@ -45,15 +48,12 @@ use codex_api::ResponsesClient as ApiResponsesClient;
 use codex_api::ResponsesOptions as ApiResponsesOptions;
 use codex_api::ResponsesWebsocketClient as ApiWebSocketResponsesClient;
 use codex_api::ResponsesWebsocketConnection as ApiWebSocketConnection;
+use codex_api::ResponsesWsRequest;
 use codex_api::SseTelemetry;
 use codex_api::TransportError;
 use codex_api::WebsocketTelemetry;
 use codex_api::build_conversation_headers;
-use codex_api::common::Reasoning;
-use codex_api::common::ResponsesWsRequest;
 use codex_api::create_text_param_for_request;
-use codex_api::error::ApiError;
-use codex_api::requests::responses::Compression;
 use codex_api::response_create_client_metadata;
 use codex_app_server_protocol::AuthMode;
 use codex_login::AuthManager;
@@ -98,8 +98,8 @@ use crate::client_common::ResponseEvent;
 use crate::client_common::ResponseStream;
 use crate::flags::CODEX_RS_SSE_FIXTURE;
 use crate::util::emit_feedback_auth_recovery_tags;
-use codex_api::api_bridge::CoreAuthProvider;
-use codex_api::api_bridge::map_api_error;
+use codex_api::CoreAuthProvider;
+use codex_api::map_api_error;
 use codex_feedback::FeedbackRequestTags;
 use codex_feedback::emit_feedback_request_tags_with_auth_env;
 use codex_login::api_bridge::auth_provider_from_auth;