[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/tui/src/lib.rs

@@ -102,7 +102,8 @@ mod clipboard_paste;
 mod clipboard_text;
 mod collaboration_modes;
 mod color;
-pub mod custom_terminal;
+pub(crate) mod custom_terminal;
+pub use custom_terminal::Terminal;
 mod cwd_prompt;
 mod debug_config;
 mod diff_render;
@@ -113,10 +114,12 @@ mod file_search;
 mod frames;
 mod get_git_diff;
 mod history_cell;
-pub mod insert_history;
+pub(crate) mod insert_history;
+pub use insert_history::insert_history_lines;
 mod key_hint;
 mod line_truncation;
-pub mod live_wrap;
+pub(crate) mod live_wrap;
+pub use live_wrap::RowBuilder;
 mod local_chatgpt_auth;
 mod markdown;
 mod markdown_render;
@@ -126,10 +129,10 @@ mod model_catalog;
 mod model_migration;
 mod multi_agents;
 mod notifications;
-pub mod onboarding;
+pub(crate) mod onboarding;
 mod oss_selection;
 mod pager_overlay;
-pub mod public_widgets;
+pub(crate) mod public_widgets;
 mod render;
 mod resume_picker;
 mod selection_list;
@@ -148,7 +151,8 @@ mod theme_picker;
 mod tooltips;
 mod tui;
 mod ui_consts;
-pub mod update_action;
+pub(crate) mod update_action;
+pub use update_action::UpdateAction;
 mod update_prompt;
 mod updates;
 mod version;
@@ -212,7 +216,7 @@ mod voice {
 mod wrapping;
 
 #[cfg(test)]
-pub mod test_backend;
+pub(crate) mod test_backend;
 #[cfg(test)]
 pub(crate) mod test_support;