[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/cloud-requirements/src/lib.rs

@@ -806,7 +806,7 @@ fn status_code_tag(status_code: Option<u16>) -> String {
 }
 
 fn emit_metric(metric_name: &str, tags: Vec<(&str, String)>) {
-    if let Some(metrics) = codex_otel::metrics::global() {
+    if let Some(metrics) = codex_otel::global() {
         let tag_refs = tags
             .iter()
             .map(|(key, value)| (*key, value.as_str()))