gitTools support for lazy auth

It would be nice to be able to do something like this:

```ts
gitTools(workspace, {
auth: async ({ command, url, remote, dir }) => {
const repo = command === "clone"
? parseRepoFromUrl(url)
: await parseRepoFromRemote(dir, remote ?? "origin");

if (!repo || !allowedRepos.has(repo)) return null;

return { token: await issueFreshInstallationToken(repo) };
},
});
```

Right now `gitTools()` accepts static `token` / `auth` when the provider is created:

https://github.com/cloudflare/agents/blob/main/packages/shell/src/git/provider.ts#L144-L162

That makes GitHub App installation tokens awkward for long-running Think agents. The token is async to issue, short-lived, and repo-scoped. We also don't want to put it in model-visible tool arguments.

It would be nice if this basically worked like git credential-helper behavior: the model calls `git.clone`, `git.fetch`, or `git.push` normally, and the runtime provides the right credential at the time of the operation.