Add provider-registry skill

[kaxil]

Adds a Claude Code skill for the af registry CLI commands (added in #145). The skill auto-activates when editing registry.py or its tests, giving Claude context about the registry API, commands, caching, and response shapes.

Skill structure

.claude/skills/provider-registry/
├── SKILL.md            — Overview, commands, architecture, jq examples
├── api-endpoints.md    — All registry endpoints with response shapes
└── caching.md          — Cache location, TTL strategy, implementation guide

What it provides to agents

• Commands reference — all four af registry commands with options
• API endpoints — full endpoint table mapping CLI commands to URLs
• Response shapes — JSON structure for each endpoint (providers, modules, parameters, connections)
• Caching details — TTL strategy (1h latest vs 30d versioned), cache location, how to add new cached commands
• jq examples — filtering by module type, extracting parameters, listing connection types