feat: add Claude Code plugin packaging and multi-app setup docs (#16)

Package the MCP server as a Claude Code plugin with slash commands,
contextual skills, and a security hook. Add configuration snippets
for 8 client environments (Claude Code, Claude Desktop, VS Code,
Cursor, Windsurf, Continue.dev, OpenCode, Open WebUI).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: John D