[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/amend.rs

@@ -9,7 +9,6 @@ use std::path::PathBuf;
 use crate::decision::Decision;
 use crate::rule::NetworkRuleProtocol;
 use crate::rule::normalize_network_rule_host;
-use serde_json;
 use thiserror::Error;
 
 #[derive(Debug, Error)]