Add durable public-URL output and URL-based chaining to xAI Grok Imagine:
- Store generated media on files-cdn with permanent public HTTPS URLs
(public_url: true, no expiry by default).
- Chain by URL: generate -> edit -> extend each take a prior result's
public HTTPS URL (or a data URI / local file for inputs).
- Add provider-specific xai_video_edit and xai_video_extend tools.
- Image generation: public-URL/storage output, multi-reference edits,
and ~/ local-path support for image edits.
Credentials use xAI Grok device-code OAuth (separate PR).
