Mark primary sources on create; create YouTube video sources

Minor Changes

• 9069be7: Add --primary to admin source create so an org's primary changelog can be marked in one step (isPrimary on the create POST), instead of creating the source and then running a follow-up admin source update <slug> --primary. The REST create endpoint and the manage_source "add" action already accepted this — the CLI was the only surface missing it, so it no longer rejects the --primary the managing-sources skill documents.
• 1072996: Add admin source create-video <channel-or-playlist-url> --org <slug> to materialize a video source from a YouTube channel/playlist (POST /v1/sources/video), surfacing the resolved provider/channel and backfilled release count. The generic source create now rejects --type video and pasted youtube.com/youtu.be URLs with a pointer to the dedicated verb — mirroring the App Store guard — so a YouTube URL can no longer be silently mis-created as an empty-bodied feed source.