chore: add metadata.json to openrouter-models, openrouter-images, and openrouter-oauth

Problem

Three of five skills are missing metadata.json:

• openrouter-models — no metadata
• openrouter-images — no metadata
• openrouter-oauth — no metadata

open-responses and openrouter-typescript-sdk both have metadata.json with version, organization, date, and abstract fields. The inconsistency means:

• Plugin managers and marketplaces can't detect when these three skills are stale
• There's no version number to bump when making breaking changes to the skill

Fix

Add metadata.json to all three missing skills, following the existing pattern:

{
  "version": "1.0.0",
  "organization": "OpenRouter Inc",
  "date": "April 2026",
  "abstract": "<one paragraph describing what the skill covers>"
}

Reviewed by Perry