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
