[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/execpolicy/src/parser.rs

@@ -1,6 +1,5 @@
 use codex_utils_absolute_path::AbsolutePathBuf;
 use multimap::MultiMap;
-use shlex;
 use starlark::any::ProvidesStaticType;
 use starlark::codemap::FileSpan;
 use starlark::environment::GlobalsBuilder;