Update skill install guide, fix language default, and improve installer

Add install script, skill file, and Desktop App setup guide

Remove model selection - only gemini-3-flash-preview available in Gemini CLI

Change default model from gemini-3-pro-preview to gemini-3-flash-preview

Revert -y flag back to --approval-mode yolo

Fix: use -y flag instead of --approval-mode for Gemini CLI compatibility

Add Claude Desktop App installation instructions

Apply --include-directories to all file-accepting tools

Add cross-platform support (macOS, Linux, Windows)

Initial release: Gemini MCP server for Claude Code