Add AGENTS.md with Cursor Cloud development instructions

[ericokuma]

Add a top-level AGENTS.md file with Cursor Cloud specific instructions for future agents to understand the development environment, startup caveats, and common commands reference.

This documents:

• How to start the local dev environment (runtime + frontend)
• Key lint/test/build commands
• Known environment caveats (vitest worker pool issues, svelte-kit sync requirements, Go module download times)
• Required tool versions (Go 1.25, Node.js 22)

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!