[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/core/src/exec.rs

@@ -417,7 +417,7 @@ fn record_windows_sandbox_spawn_failure(
     } else {
         "legacy"
     };
-    if let Some(metrics) = codex_otel::metrics::global() {
+    if let Some(metrics) = codex_otel::global() {
         let _ = metrics.counter(
             "codex.windows_sandbox.createprocessasuserw_failed",
             /*inc*/ 1,