drafts update: new text drops attached media; X length check runs on LinkedIn-only drafts; no unschedule #2

Three things found while editing two LinkedIn drafts on 2026-09-17.

1. drafts update <id> -f new.txt rebuilds the platforms block and DROPS the media that was attached at create time. Expected: keep existing media_ids unless --media/--media-file is passed. Workaround: a second drafts update <id> --append-media --media-file <path>.

2. drafts update with text applies the X 280 weighted check even when the draft is LinkedIn-only (created with -p linkedin). Expected: read the platform from the draft, or from -p, before checking. Workaround: -p linkedin --no-length-check.

3. There is no way to unschedule. PATCH /v2/social-sets/<set>/drafts/<id> with {"publish_at": null} returns the draft to draft; a drafts unschedule <id> (or update --unschedule) would wrap it.