README: add vault key naming schema (<server-name>-<verwendungszweck>,
kebab-case) and a key-exception example to the mcp/ guard rules.
Reference fragments in mcp/: zugferd-service.yaml (OAuth standard case,
no credential field) and az-zoll-service.yaml (documented key exception
using ${VAULT:az-zoll-service-api-key} placeholder) — neither contains
any secret.
Five invalid mcp fixtures under tests/fixtures/mcp/invalid/ covering all
README-documented failure modes: inline secret, missing server-name
fragment structure, missing url, missing type, wrong placeholder syntax.
Closes az-agent-defaults-95y
