fix: remove overly broad "codex" keyword from openai_codex provider

The bare keyword "codex" causes false positive matches when any model
name happens to contain "codex" (e.g. "gpt-5.3-codex" on a custom
provider).  This incorrectly routes the request through the OAuth-based
OpenAI Codex provider, producing "OAuth credentials not found" errors
even when a valid custom api_key and api_base are configured.

Keep only the explicit "openai-codex" keyword so that auto-detection
requires the canonical prefix.  Users can still set provider: "custom"
to force the custom endpoint, but auto-detection should not collide.

Closes #1311

nanobot/providers/registry.py

@@ -201,7 +201,7 @@ PROVIDERS: tuple[ProviderSpec, ...] = (
     # OpenAI Codex: uses OAuth, not API key.
     ProviderSpec(
         name="openai_codex",
-        keywords=("openai-codex", "codex"),
+        keywords=("openai-codex",),
         env_key="",                         # OAuth-based, no API key
         display_name="OpenAI Codex",
         litellm_prefix="",                  # Not routed through LiteLLM