[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/bottom_pane/feedback_view.rs

@@ -1,5 +1,5 @@
-use codex_feedback::feedback_diagnostics::FEEDBACK_DIAGNOSTICS_ATTACHMENT_FILENAME;
-use codex_feedback::feedback_diagnostics::FeedbackDiagnostics;
+use codex_feedback::FEEDBACK_DIAGNOSTICS_ATTACHMENT_FILENAME;
+use codex_feedback::FeedbackDiagnostics;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
 use crossterm::event::KeyModifiers;
@@ -556,7 +556,7 @@ mod tests {
     use super::*;
     use crate::app_event::AppEvent;
     use crate::app_event_sender::AppEventSender;
-    use codex_feedback::feedback_diagnostics::FeedbackDiagnostic;
+    use codex_feedback::FeedbackDiagnostic;
     use pretty_assertions::assert_eq;
 
     fn render(view: &FeedbackNoteView, width: u16) -> String {

codex-rs/tui/src/bottom_pane/mod.rs

@@ -76,7 +76,7 @@ pub(crate) struct MentionBinding {
 mod chat_composer;
 mod chat_composer_history;
 mod command_popup;
-pub mod custom_prompt_view;
+pub(crate) mod custom_prompt_view;
 mod experimental_features_view;
 mod file_search_popup;
 mod footer;
@@ -108,7 +108,7 @@ pub(crate) use title_setup::TerminalTitleSetupView;
 mod paste_burst;
 mod pending_input_preview;
 mod pending_thread_approvals;
-pub mod popup_consts;
+pub(crate) mod popup_consts;
 mod scroll_state;
 mod selection_popup_common;
 mod textarea;