[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/models-manager/src/lib.rs

@@ -1,6 +1,6 @@
-pub mod cache;
+pub(crate) mod cache;
 pub mod collaboration_mode_presets;
-pub mod config;
+pub(crate) mod config;
 pub mod manager;
 pub mod model_info;
 pub mod model_presets;