[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/cloud-tasks/src/scrollable_diff.rs

@@ -86,11 +86,11 @@ impl ScrollableDiff {
         self.scroll_by(delta);
     }
 
-    pub fn to_top(&mut self) {
+    pub fn scroll_to_top(&mut self) {
         self.state.scroll = 0;
     }
 
-    pub fn to_bottom(&mut self) {
+    pub fn scroll_to_bottom(&mut self) {
         self.state.scroll = self.max_scroll();
     }