Repo buttons

Add a repo query parameter to https://exe.dev/new to spin up a VM, open Shelley, clone the repo, and prefill the deploy instructions:

/new?repo=https://github.com/OWNER/REPO

Shelley will use repo instructions like AGENTS.md when available.

Public repos can add our buttons to their README.md:

[![Deploy on exe.dev](https://raw.githubusercontent.com/boldsoftware/exe.dev/main/assets/buttons/deploy-on-exe-dev.png)](https://exe.dev/new?repo=https://github.com/OWNER/REPO)

[![Build with Shelley](https://raw.githubusercontent.com/boldsoftware/exe.dev/main/assets/buttons/build-with-shelley.png)](https://exe.dev/new?repo=https://github.com/OWNER/REPO)