Add Model Selection Guide for Agentic Development (Opus 5 context-engineering shift) Covers Anthropic's July 24 finding: 80% of Claude Code's system prompt removed for Claude 5 models with no performance loss. Task-type routing (IaC -> Opus 4.8; agentic loops -> Opus 5; frontier -> Fable 5), instruction-file simplification strategy, our use-case mappings, and community best practices.