[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/turn_timing.rs

@@ -3,8 +3,8 @@ use std::time::Instant;
 use std::time::SystemTime;
 use std::time::UNIX_EPOCH;
 
-use codex_otel::metrics::names::TURN_TTFM_DURATION_METRIC;
-use codex_otel::metrics::names::TURN_TTFT_DURATION_METRIC;
+use codex_otel::TURN_TTFM_DURATION_METRIC;
+use codex_otel::TURN_TTFT_DURATION_METRIC;
 use codex_protocol::items::TurnItem;
 use codex_protocol::models::ResponseItem;
 use tokio::sync::Mutex;