[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/app-server/src/codex_message_processor/plugin_mcp_oauth.rs

@@ -5,10 +5,10 @@ use codex_app_server_protocol::McpServerOauthLoginCompletedNotification;
 use codex_app_server_protocol::ServerNotification;
 use codex_config::types::McpServerConfig;
 use codex_core::config::Config;
-use codex_mcp::mcp::auth::McpOAuthLoginSupport;
-use codex_mcp::mcp::auth::oauth_login_support;
-use codex_mcp::mcp::auth::resolve_oauth_scopes;
-use codex_mcp::mcp::auth::should_retry_without_scopes;
+use codex_mcp::McpOAuthLoginSupport;
+use codex_mcp::oauth_login_support;
+use codex_mcp::resolve_oauth_scopes;
+use codex_mcp::should_retry_without_scopes;
 use codex_rmcp_client::perform_oauth_login_silent;
 use tracing::warn;