[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/manager_metrics.rs

@@ -2,9 +2,9 @@ use crate::harness::attributes_to_map;
 use crate::harness::build_metrics_with_defaults;
 use crate::harness::find_metric;
 use crate::harness::latest_metrics;
+use codex_otel::Result;
 use codex_otel::SessionTelemetry;
 use codex_otel::TelemetryAuthMode;
-use codex_otel::metrics::Result;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
 use opentelemetry_sdk::metrics::data::AggregatedMetrics;