[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/codex-api/src/requests/mod.rs

@@ -1,2 +1,5 @@
 pub(crate) mod headers;
-pub mod responses;
+pub(crate) mod responses;
+
+pub use responses::Compression;
+pub(crate) use responses::attach_item_ids;