[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/protocol/src/models.rs

@@ -27,7 +27,7 @@ use crate::user_input::UserInput;
 use codex_execpolicy::Policy;
 use codex_git_utils::GhostCommit;
 use codex_utils_absolute_path::AbsolutePathBuf;
-use codex_utils_image::error::ImageProcessingError;
+use codex_utils_image::ImageProcessingError;
 use schemars::JsonSchema;
 
 use crate::mcp::CallToolResult;