Add MCP server for AI agent integration

[pdhruv93]

Summary

• Add @opensplit/mcp package — an MCP (Model Context Protocol) server that lets AI agents manage expenses, groups, and friends through natural language
• Supports both stdio (for Claude Desktop, Claude Code, Cursor) and HTTP (for multi-user/hosted deployments) transports
• 13 tools registered: CRUD for expenses, groups, and friends, plus reference tools (current user, currencies, categories)
• Dockerized alongside the API via docker-compose.yml
• Updated README with full MCP setup guide for both transport modes

Tools

Domain | Tools -- | -- Expenses | create_expense, list_expenses, get_expense, update_expense, delete_expense Groups | list_groups, get_group, create_group, add_group_member Friends | list_friends, get_friend, add_friend Reference | get_current_user, list_currencies, list_categories

Test plan

•  pnpm --filter @opensplit/mcp build succeeds
•  Stdio mode: configure in Claude Desktop/Code, verify tools are listed and an expense can be created
•  HTTP mode: docker compose up, POST to /mcp with Bearer token, verify tool calls work
•  Verify error responses for missing auth (401) and invalid endpoints (404)