fix(README.md): add language to fenced code block (#1)

markdownlint MD040 was failing on the Codex example block.
Use 'text' since the content is ASCII art, not source code.

Generated by Coder Agents.

Author: kylecarbs

README.md

@@ -21,7 +21,7 @@ claude plugin install coder-skills@coder-skills
 
 After installing, ask your agent to install or set up Coder:
 
-```
+```text
 ╭────────────────────────────────────────────────────╮
 │ >_ OpenAI Codex (v0.130.0)                         │
 │                                                    │