Repo buttons

Add "Deploy on exe.dev" and "Build with Shelley" buttons to your README

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`:

```markdown
[![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)
```