[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/otel_init.rs

@@ -3,11 +3,11 @@ use codex_config::types::OtelExporterKind as Kind;
 use codex_config::types::OtelHttpProtocol as Protocol;
 use codex_features::Feature;
 use codex_login::default_client::originator;
+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::config::OtelTlsConfig as OtelTlsSettings;
+use codex_otel::OtelSettings;
+use codex_otel::OtelTlsConfig as OtelTlsSettings;
 use std::error::Error;
 
 /// Build an OpenTelemetry provider from the app Config.