[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/core/src/external_agent_config.rs

@@ -676,7 +676,7 @@ fn emit_migration_metric(
     item_type: ExternalAgentConfigMigrationItemType,
     skills_count: Option<usize>,
 ) {
-    let Some(metrics) = codex_otel::metrics::global() else {
+    let Some(metrics) = codex_otel::global() else {
         return;
     };
     let tags = migration_metric_tags(item_type, skills_count);