[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/path-utils/src/lib.rs

@@ -1,6 +1,8 @@
 //! Path normalization, symlink resolution, and atomic writes shared across Codex crates.
 
-pub mod env;
+pub(crate) mod env;
+pub use env::is_headless_environment;
+pub use env::is_wsl;
 
 use codex_utils_absolute_path::AbsolutePathBuf;
 use std::collections::HashSet;