[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/otel/tests/suite/send.rs

@@ -3,7 +3,7 @@ use crate::harness::build_metrics_with_defaults;
 use crate::harness::find_metric;
 use crate::harness::histogram_data;
 use crate::harness::latest_metrics;
-use codex_otel::metrics::Result;
+use codex_otel::Result;
 use pretty_assertions::assert_eq;
 use std::collections::BTreeMap;