Handle expired agreements and rewrite README

- Implement robust End User Agreement expiry detection and handling
- Add graceful error recovery for failed accounts
- Rewrite README.md to focus on user benefits
- Add documentation guidelines to AGENTS.md

Cargo.lock

@@ -174,6 +174,7 @@ dependencies = [
  "serde",
  "serde_json",
  "task-local-extensions",
+ "thiserror",
  "tokio",
  "tracing",
  "tracing-subscriber",