[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/chatwidget/tests/popups_and_settings.rs

@@ -1761,13 +1761,11 @@ async fn feedback_upload_consent_popup_snapshot() {
         chat.app_event_tx.clone(),
         crate::app_event::FeedbackCategory::Bug,
         chat.current_rollout_path.clone(),
-        &codex_feedback::feedback_diagnostics::FeedbackDiagnostics::new(vec![
-            codex_feedback::feedback_diagnostics::FeedbackDiagnostic {
-                headline: "Proxy environment variables are set and may affect connectivity."
-                    .to_string(),
-                details: vec!["HTTPS_PROXY = hello".to_string()],
-            },
-        ]),
+        &codex_feedback::FeedbackDiagnostics::new(vec![codex_feedback::FeedbackDiagnostic {
+            headline: "Proxy environment variables are set and may affect connectivity."
+                .to_string(),
+            details: vec!["HTTPS_PROXY = hello".to_string()],
+        }]),
     ));
 
     let popup = render_bottom_popup(&chat, /*width*/ 80);
@@ -1782,13 +1780,11 @@ async fn feedback_good_result_consent_popup_includes_connectivity_diagnostics_fi
         chat.app_event_tx.clone(),
         crate::app_event::FeedbackCategory::GoodResult,
         chat.current_rollout_path.clone(),
-        &codex_feedback::feedback_diagnostics::FeedbackDiagnostics::new(vec![
-            codex_feedback::feedback_diagnostics::FeedbackDiagnostic {
-                headline: "Proxy environment variables are set and may affect connectivity."
-                    .to_string(),
-                details: vec!["HTTPS_PROXY = hello".to_string()],
-            },
-        ]),
+        &codex_feedback::FeedbackDiagnostics::new(vec![codex_feedback::FeedbackDiagnostic {
+            headline: "Proxy environment variables are set and may affect connectivity."
+                .to_string(),
+            details: vec!["HTTPS_PROXY = hello".to_string()],
+        }]),
     ));
 
     let popup = render_bottom_popup(&chat, /*width*/ 80);