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>

docs/docs.json

@@ -32,7 +32,6 @@
             "group": "LLM Providers",
             "pages": [
               "llm-providers/overview",
-              "llm-providers/models",
               "llm-providers/openai",
               "llm-providers/anthropic",
               "llm-providers/openrouter",