feat: Add AGENTS.md for setup instructions and MCP tools

Commit notes

This commit introduces a new AGENTS.md file that provides setup commands, code style guidelines, and details on available MCP tools for Svelte and SvelteKit. The document includes instructions for installing dependencies, starting the development server, and running tests, as well as outlining the use of tools like list-sections, get-documentation, svelte-autofixer, and playground-link to enhance user experience and documentation access.