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>

strix/llm/utils.py

@@ -36,7 +36,7 @@ STRIX_MODEL_MAP: dict[str, str] = {
     "claude-opus-4.6": "anthropic/claude-opus-4-6",
     "gpt-5.2": "openai/gpt-5.2",
     "gpt-5.1": "openai/gpt-5.1",
-    "gpt-5": "openai/gpt-5",
+    "gpt-5.4": "openai/gpt-5.4",
     "gemini-3-pro-preview": "gemini/gemini-3-pro-preview",
     "gemini-3-flash-preview": "gemini/gemini-3-flash-preview",
     "glm-5": "openrouter/z-ai/glm-5",