[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

AGENTS.md

@@ -21,6 +21,7 @@ In the codex-rs folder where the rust code lives:
 - Newly added traits should include doc comments that explain their role and how implementations are expected to use them.
 - When writing tests, prefer comparing the equality of entire objects over fields one by one.
 - When making a change that adds or changes an API, ensure that the documentation in the `docs/` folder is up to date if applicable.
+- Prefer private modules and explicitly exported public crate API.
 - If you change `ConfigToml` or nested config types, run `just write-config-schema` to update `codex-rs/core/config.schema.json`.
 - If you change Rust dependencies (`Cargo.toml` or `Cargo.lock`), run `just bazel-lock-update` from the
   repo root to refresh `MODULE.bazel.lock`, and include that lockfile update in the same change.