feat: readiness tool (#4090)

Readiness flag with token-based subscription and async wait function
that waits for all the subscribers to be ready

codex-rs/common/src/lib.rs

@@ -34,3 +34,5 @@ pub mod model_presets;
 // Shared approval presets (AskForApproval + Sandbox) used by TUI and MCP server
 // Not to be confused with AskForApproval, which we should probably rename to EscalationPolicy.
 pub mod approval_presets;
+// Readiness flag with token-based authorization and async waiting (Tokio).
+pub mod readiness;