[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/otlp_http_loopback.rs

@@ -1,10 +1,10 @@
+use codex_otel::MetricsClient;
+use codex_otel::MetricsConfig;
+use codex_otel::OtelExporter;
+use codex_otel::OtelHttpProtocol;
 use codex_otel::OtelProvider;
-use codex_otel::config::OtelExporter;
-use codex_otel::config::OtelHttpProtocol;
-use codex_otel::config::OtelSettings;
-use codex_otel::metrics::MetricsClient;
-use codex_otel::metrics::MetricsConfig;
-use codex_otel::metrics::Result;
+use codex_otel::OtelSettings;
+use codex_otel::Result;
 use std::collections::HashMap;
 use std::io::Read as _;
 use std::io::Write as _;