[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/tests/suite/vt100_history.rs

@@ -22,20 +22,20 @@ macro_rules! assert_contains {
 }
 
 struct TestScenario {
-    term: codex_tui::custom_terminal::Terminal<VT100Backend>,
+    term: codex_tui::Terminal<VT100Backend>,
 }
 
 impl TestScenario {
     fn new(width: u16, height: u16, viewport: Rect) -> Self {
         let backend = VT100Backend::new(width, height);
-        let mut term = codex_tui::custom_terminal::Terminal::with_options(backend)
-            .expect("failed to construct terminal");
+        let mut term =
+            codex_tui::Terminal::with_options(backend).expect("failed to construct terminal");
         term.set_viewport_area(viewport);
         Self { term }
     }
 
     fn run_insert(&mut self, lines: Vec<Line<'static>>) {
-        codex_tui::insert_history::insert_history_lines(&mut self.term, lines)
+        codex_tui::insert_history_lines(&mut self.term, lines)
             .expect("Failed to insert history lines in test");
     }
 }