Rename the Codex CLI entry's model label back to GPT-5.5

There is no "GPT-5.5 Codex" model — Codex is the harness. The entry is
GPT-5.5 run in Codex CLI at xhigh effort, which the harness and effort
fields already record; READMEs and code comments updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

models/gpt-5-5-xhigh.json

@@ -1,5 +1,5 @@
 {
-  "label": "GPT-5.5 Codex",
+  "label": "GPT-5.5",
   "vendor": "OpenAI",
   "harness": "Codex CLI",
   "effort": "xhigh",