[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/utils/pty/src/win/mod.rs

@@ -44,7 +44,7 @@ use winapi::um::processthreadsapi::*;
 use winapi::um::synchapi::WaitForSingleObject;
 use winapi::um::winbase::INFINITE;
 
-pub mod conpty;
+pub(crate) mod conpty;
 mod procthreadattr;
 mod psuedocon;