[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-backend-openapi-models/src/models/mod.rs

@@ -4,41 +4,41 @@
 // The process for this will change
 
 // Config
-pub mod config_file_response;
+pub(crate) mod config_file_response;
 pub use self::config_file_response::ConfigFileResponse;
 
 // Cloud Tasks
-pub mod code_task_details_response;
+pub(crate) mod code_task_details_response;
 pub use self::code_task_details_response::CodeTaskDetailsResponse;
 
-pub mod task_response;
+pub(crate) mod task_response;
 pub use self::task_response::TaskResponse;
 
-pub mod external_pull_request_response;
+pub(crate) mod external_pull_request_response;
 pub use self::external_pull_request_response::ExternalPullRequestResponse;
 
-pub mod git_pull_request;
+pub(crate) mod git_pull_request;
 pub use self::git_pull_request::GitPullRequest;
 
-pub mod task_list_item;
+pub(crate) mod task_list_item;
 pub use self::task_list_item::TaskListItem;
 
-pub mod paginated_list_task_list_item_;
+pub(crate) mod paginated_list_task_list_item_;
 pub use self::paginated_list_task_list_item_::PaginatedListTaskListItem;
 
 // Rate Limits
-pub mod additional_rate_limit_details;
+pub(crate) mod additional_rate_limit_details;
 pub use self::additional_rate_limit_details::AdditionalRateLimitDetails;
 
-pub mod rate_limit_status_payload;
+pub(crate) mod rate_limit_status_payload;
 pub use self::rate_limit_status_payload::PlanType;
 pub use self::rate_limit_status_payload::RateLimitStatusPayload;
 
-pub mod rate_limit_status_details;
+pub(crate) mod rate_limit_status_details;
 pub use self::rate_limit_status_details::RateLimitStatusDetails;
 
-pub mod rate_limit_window_snapshot;
+pub(crate) mod rate_limit_window_snapshot;
 pub use self::rate_limit_window_snapshot::RateLimitWindowSnapshot;
 
-pub mod credit_status_details;
+pub(crate) mod credit_status_details;
 pub use self::credit_status_details::CreditStatusDetails;