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>

scripts/install.sh

@@ -335,14 +335,11 @@ echo -e "${MUTED}  AI Penetration Testing Agent${NC}"
 echo ""
 echo -e "${MUTED}To get started:${NC}"
 echo ""
-echo -e "  ${CYAN}1.${NC} Get your Strix API key:"
-echo -e "     ${MUTED}https://models.strix.ai${NC}"
-echo ""
-echo -e "  ${CYAN}2.${NC} Set your environment:"
+echo -e "  ${CYAN}1.${NC} Set your environment:"
 echo -e "     ${MUTED}export LLM_API_KEY='your-api-key'${NC}"
-echo -e "     ${MUTED}export STRIX_LLM='strix/gpt-5'${NC}"
+echo -e "     ${MUTED}export STRIX_LLM='openai/gpt-5.4'${NC}"
 echo ""
-echo -e "  ${CYAN}3.${NC} Run a penetration test:"
+echo -e "  ${CYAN}2.${NC} Run a penetration test:"
 echo -e "     ${MUTED}strix --target https://example.com${NC}"
 echo ""
 echo -e "${MUTED}For more information visit ${NC}https://strix.ai"