[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/utils/image/src/lib.rs

@@ -2,7 +2,6 @@ use std::num::NonZeroUsize;
 use std::path::Path;
 use std::sync::LazyLock;
 
-use crate::error::ImageProcessingError;
 use base64::Engine;
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
 use codex_utils_cache::BlockingLruCache;
@@ -21,7 +20,9 @@ pub const MAX_WIDTH: u32 = 2048;
 /// Maximum height used when resizing images before uploading.
 pub const MAX_HEIGHT: u32 = 768;
 
-pub mod error;
+pub(crate) mod error;
+
+pub use crate::error::ImageProcessingError;
 
 #[derive(Debug, Clone)]
 pub struct EncodedImage {