feat: add CoinGecko CLI agent skill

[hungv47]

• Add a coingecko-cli skill under skills/ for proactive agent use of the cg CLI and also let users install via npx skills add coingecko/coingecko-cli
• Document command resolution, auth handling, JSON-first usage, dry-run behavior, and paid endpoint guardrails
• Add a compact command reference for prices, search, markets, trending, history, top movers, watch, TUI, and maintenance commands

Validation:

• Ran quick_validate.py against skills/coingecko-cli
• Parsed SKILL.md frontmatter and agents/openai.yaml as valid YAML