docs: restore AGENTS context routing rules

[xnoto]

Summary

• restore the root AGENTS.md guidance that tells non-llama agents to load the context-mode and context7 skills at session start
• add the context-mode routing rules for large-output commands, web fetches, and analysis-oriented file reads
• add the Context7 documentation-routing rules plus the AWS/Terraform/OpenTofu/OpenCode exceptions and their specialized tool fallbacks

Validation

• not run; documentation-only change