[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/onboarding/mod.rs

@@ -1,6 +1,5 @@
 mod auth;
-pub mod onboarding_screen;
+pub(crate) mod onboarding_screen;
 mod trust_directory;
 pub(crate) use auth::mark_url_hyperlink;
-pub use trust_directory::TrustDirectorySelection;
 mod welcome;