feat(mcp): reference fragments + vault key naming schema + guard fixtures

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

tests/fixtures/mcp/invalid/ohne-server-name.yaml

@@ -0,0 +1,5 @@
+# Testgegenstand: keine Fragment-Struktur — der Server-Name fehlt als Schlüssel,
+# die Felder stehen auf Root-Ebene statt unter genau einem Server-Namen.
+type: remote
+url: https://mcp.example.az.local/ohne-namen
+enabled: true