Document inferred empty response shapes

[xav-db]

Summary

• document null for empty at-most-one returns and [] for empty collections, folds, and mutations
• clarify that populated response shapes do not change and no declared returns produces {}
• update HQL FIRST guidance for the populated and empty cases
• add current v3 agent metadata and concise navigation for every skill
• remove duplicated canonical examples from skill entrypoints while keeping them in EXAMPLES.md
• add the missing Rust and TypeScript summaries to the repository README

Impact

Agents get the current POST /v2/query response contract without reintroducing legacy v1, stored-route, or bundle guidance.

Validation

• all skill packages pass quick_validate.py
• all agents/openai.yaml files parse successfully
• metadata prompts match their skill names and current v3 terminology
• git diff --check
• independent read-only consistency review