Document Codex CLI auth prerequisite in README

Author: KSemenenko

AGENTS.md

@@ -174,6 +174,7 @@ If no new rule is detected -> do not update the file.
 - For this project, remove legacy/compatibility shims immediately (including `[Obsolete]` bridges and duplicate old properties); keep only the current API surface.
 - README first examples must be beginner-friendly: avoid advanced/optional knobs (for example `CodexExecutablePath`) in the very first snippet.
 - README in this repository must describe only current first-version product behavior; do not add migration/legacy compatibility sections or old-namespace guidance.
+- README must explicitly state runtime prerequisites: `codex` CLI installed and user already authenticated (`codex login`) before SDK usage.
 - When a README snippet shows model tuning, include `ModelReasoningEffort` together with `Model`.
 - Public examples should build output schemas with typed `StructuredOutputSchema` models and map responses to typed DTOs for readability and maintainability.
 - Do not keep or add `JsonSchema` helper abstractions in SDK API/tests; use typed request/response DTO models instead of schema-builder utilities.

README.md

@@ -24,6 +24,19 @@ All consumer usage examples are documented in this README; this repository inten
 dotnet add package ManagedCode.CodexSharpSDK
 ```
 
+## Prerequisites
+
+Before using this SDK, you must have:
+- `codex` CLI installed and available in `PATH`
+- an already authenticated Codex session (`codex login`)
+
+Quick check:
+
+```bash
+codex --version
+codex login
+```
+
 ## Quickstart
 
 ```csharp