Clarify provider setup in README (#1406)

Author: Ivorisnoob

README.md

@@ -5,7 +5,11 @@ T3 Code is a minimal web GUI for coding agents (currently Codex and Claude, more
 ## How to use
 
 > [!WARNING]
-> You need to have [Codex CLI](https://github.com/openai/codex) installed and authorized for T3 Code to work.
+> T3 Code currently supports Codex and Claude.
+> Install and authenticate at least one provider before use:
+>
+> - Codex: install [Codex CLI](https://github.com/openai/codex) and run `codex login`
+> - Claude: install Claude Code and run `claude auth login`
 
 ```bash
 npx t3