feat(skills): az-hilfe onboarding skill + invalid skill guard fixtures

Replace the ow-hello pilot skill (never rolled out, dropped before first
commit) with az-hilfe — a German onboarding/help router skill for AZ
users, modeled after the ask-matt pattern: explains the four artifact
types, typical user questions with answer patterns, contribution path
via IT/repo, and honest rollout status (commands/agents/mcp pending).

Add five invalid skill fixtures under tests/fixtures/skills/invalid/,
each failing exactly one README guard rule: missing SKILL.md, missing
frontmatter opener, empty frontmatter, name != folder, missing
description. Remove obsolete .gitkeep placeholders.

Closes az-agent-defaults-dzx, az-agent-defaults-1ub

tests/fixtures/skills/invalid/ohne-description/SKILL.md

@@ -0,0 +1,8 @@
+---
+name: ohne-description
+---
+
+# Skill ohne Description
+
+Dieses Frontmatter enthält `name`, aber das Pflichtfeld `description` fehlt.
+Der Guard muss dieses Artefakt rot abbrechen.