[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/feedback/src/lib.rs

@@ -14,8 +14,6 @@ use anyhow::anyhow;
 use codex_login::AuthEnvTelemetry;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
-use feedback_diagnostics::FEEDBACK_DIAGNOSTICS_ATTACHMENT_FILENAME;
-use feedback_diagnostics::FeedbackDiagnostics;
 use tracing::Event;
 use tracing::Level;
 use tracing::field::Visit;
@@ -24,7 +22,10 @@ use tracing_subscriber::filter::Targets;
 use tracing_subscriber::fmt::writer::MakeWriter;
 use tracing_subscriber::registry::LookupSpan;
 
-pub mod feedback_diagnostics;
+pub(crate) mod feedback_diagnostics;
+pub use feedback_diagnostics::FEEDBACK_DIAGNOSTICS_ATTACHMENT_FILENAME;
+pub use feedback_diagnostics::FeedbackDiagnostic;
+pub use feedback_diagnostics::FeedbackDiagnostics;
 
 const DEFAULT_MAX_BYTES: usize = 4 * 1024 * 1024; // 4 MiB
 const SENTRY_DSN: &str =
@@ -609,7 +610,7 @@ mod tests {
     use std::fs;
 
     use super::*;
-    use feedback_diagnostics::FeedbackDiagnostic;
+    use crate::FeedbackDiagnostic;
     use pretty_assertions::assert_eq;
     use tracing_subscriber::layer::SubscriberExt;
     use tracing_subscriber::util::SubscriberInitExt;