[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/validation.rs

@@ -1,7 +1,7 @@
-use codex_otel::metrics::MetricsClient;
-use codex_otel::metrics::MetricsConfig;
-use codex_otel::metrics::MetricsError;
-use codex_otel::metrics::Result;
+use codex_otel::MetricsClient;
+use codex_otel::MetricsConfig;
+use codex_otel::MetricsError;
+use codex_otel::Result;
 use opentelemetry_sdk::metrics::InMemoryMetricExporter;
 
 fn build_in_memory_client() -> Result<MetricsClient> {