append mode

it would be super useful if we could write to a file through the api in append mode, this would allow other use cases like very basic logging without having to read and re-write the entire file

```
await files.append('my-static-app/users.log', 'created new user `${newUserId}`')
```

Start at the `files.append` API shown in the issue and trace the existing file-write path in the SDK. Define completion as appending content to an existing cloud-stored file without requiring callers to read and rewrite the entire file, while preserving the demonstrated usage.

Written by the indexing model from the issue text.