[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/codex-api/src/endpoint/realtime_websocket/methods.rs

@@ -625,9 +625,9 @@ fn normalize_realtime_path(url: &mut Url) {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::endpoint::realtime_websocket::protocol::RealtimeHandoffRequested;
     use crate::endpoint::realtime_websocket::protocol::RealtimeTranscriptDelta;
     use crate::endpoint::realtime_websocket::protocol::RealtimeTranscriptEntry;
+    use codex_protocol::protocol::RealtimeHandoffRequested;
     use codex_protocol::protocol::RealtimeInputAudioSpeechStarted;
     use codex_protocol::protocol::RealtimeResponseCancelled;
     use http::HeaderValue;