[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/state/service.rs

@@ -17,7 +17,7 @@ use codex_analytics::AnalyticsEventsClient;
 use codex_exec_server::Environment;
 use codex_hooks::Hooks;
 use codex_login::AuthManager;
-use codex_mcp::mcp_connection_manager::McpConnectionManager;
+use codex_mcp::McpConnectionManager;
 use codex_models_manager::manager::ModelsManager;
 use codex_otel::SessionTelemetry;
 use codex_rollout::state_db::StateDbHandle;