[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/render/mod.rs

@@ -1,8 +1,8 @@
 use ratatui::layout::Rect;
 
-pub mod highlight;
-pub mod line_utils;
-pub mod renderable;
+pub(crate) mod highlight;
+pub(crate) mod line_utils;
+pub(crate) mod renderable;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Insets {