[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/cli/src/wsl_paths.rs

@@ -1,7 +1,7 @@
 use std::ffi::OsStr;
 
 /// Returns true if the current process is running under WSL.
-pub use codex_utils_path::env::is_wsl;
+pub use codex_utils_path::is_wsl;
 
 /// Convert a Windows absolute path (`C:\foo\bar` or `C:/foo/bar`) to a WSL mount path (`/mnt/c/foo/bar`).
 /// Returns `None` if the input does not look like a Windows drive path.