feat(agents): reference primary agent + read-only subagent + guard fixtures

az-beitrag (mode: primary): Fachbereichs-Contributor-Assistent guiding
users to draft valid artifacts per README guard rules; keeps full
delegation capability (delegates the formal check to az-pruefer via
task tool) and explicitly acknowledges the managed permission layer.

az-pruefer (mode: subagent): read-only checker with tools [read, glob,
grep] — no edit/bash; invocable via @mention and task tool.

Three invalid agent fixtures under tests/fixtures/agents/invalid/
covering all README-documented failure modes: missing mode, missing
description, invalid mode value.

Closes az-agent-defaults-gyj

tests/fixtures/agents/invalid/ungueltiger-mode.md

@@ -0,0 +1,7 @@
+---
+description: Testgegenstand — mode mit ungültigem Wert.
+mode: sideload
+---
+
+Der Wert von `mode` ist weder `primary` noch `subagent`. Der Guard muss rot
+abbrechen.