v1.1.3: Gemini CLI extension, 15 skills

Adds gemini-extension.json so the plugin installs as a Gemini CLI extension (gemini extensions install https://github.com/posteverywhere/claude-plugin). Connects the hosted PostEverywhere MCP server (https://mcp.posteverywhere.ai/mcp); you sign in to PostEverywhere in your browser the first time. 15 social media skills for Claude Code, Codex, Cursor, Gemini CLI and Grok.