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/Cargo.lock

@@ -316,6 +316,17 @@ dependencies = [
  "syn 2.0.104",
 ]
 
+[[package]]
+name = "async-trait"
+version = "0.1.89"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9035ad2d096bed7955a320ee7e2230574d28fd3c3a0f186cbea1ff3c7eed5dbb"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.104",
+]
+
 [[package]]
 name = "atomic-waker"
 version = "1.1.2"
@@ -653,10 +664,13 @@ dependencies = [
 name = "codex-common"
 version = "0.0.0"
 dependencies = [
+ "async-trait",
  "clap",
  "codex-core",
  "codex-protocol",
  "serde",
+ "thiserror 2.0.16",
+ "tokio",
  "toml",
 ]