chore: update default model to gpt-5.4 and remove Strix Router from docs

- Change default model from gpt-5 to gpt-5.4 across docs, tests, and examples
- Remove Strix Router references from docs, quickstart, overview, and README
- Delete models.mdx (Strix Router page) and its nav entry
- Simplify install script to suggest openai/ prefix directly
- Keep strix/ model routing support intact in code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/llm/test_llm_otel.py

@@ -1,15 +1,16 @@
 import litellm
+import pytest
 
 from strix.llm.config import LLMConfig
 from strix.llm.llm import LLM
 
 
-def test_llm_does_not_modify_litellm_callbacks(monkeypatch) -> None:
+def test_llm_does_not_modify_litellm_callbacks(monkeypatch: pytest.MonkeyPatch) -> None:
     monkeypatch.setenv("STRIX_TELEMETRY", "1")
     monkeypatch.setenv("STRIX_OTEL_TELEMETRY", "1")
     monkeypatch.setattr(litellm, "callbacks", ["custom-callback"])
 
-    llm = LLM(LLMConfig(model_name="openai/gpt-5"), agent_name=None)
+    llm = LLM(LLMConfig(model_name="openai/gpt-5.4"), agent_name=None)
 
     assert llm is not None
     assert litellm.callbacks == ["custom-callback"]