Expose backend discovery/capabilities for workspace.runtime.exec

I was reading through the README and trying to think about how an agent would safely choose between registered backends. `workspace.runtime.exec(source, { backend })` is a nice single entry point, but the README notes that the selected backend determines whether `source` is a shell command or an ECMAScript module.

From the caller's perspective, it looks like you need to already know that one backend ID is shell-like and another is JS-like. If a Workspace has multiple backends registered, there doesn't seem to be a documented way to ask the runtime what backends exist, whether they're connected/lazy, or what kind of source/result they accept.

This becomes a papercut when the caller is an agent or a generic UI: it can easily pick a backend ID and pass the wrong source type, then fail only when the backend connects/executes. A small introspection API would make this much easier to use, something like:

```ts
const backends = await workspace.runtime.listBackends();
// [{ id: 'container', kind: 'shell', ready: false }, { id: 'js', kind: 'module', ready: true }]
```

or even just:

```ts
const info = await workspace.runtime.describeBackend('container');
if (info.source === 'shell') { ... }
```

That would also give a natural place to expose backend-specific limits later, such as network access, filesystem write access, or allowed imports, without changing `exec` itself. If this already exists somewhere, it might be worth calling it out in the README because it's key to using multiple backends safely.