Moving Linear screenshots into GitHub issues: download fast, host on a branch

Linear upload URLs from list_comments are signed and expire about 5 minutes after the call, so fetch the comments and curl the images in the same step. GitHub has no API for issue image attachments (user-attachments), so a workable route is: create a branch (e.g. qa-assets) via gh api repos/O/R/git/refs, PUT each PNG with the contents API (-f content="$(base64 -i file)" -f branch=...), and embed https://raw.githubusercontent.com/O/R/<branch>/<path> in the issue body (public repos only). Also in zsh, ${3:+--assignee $3} is not word-split, so gh gets one argument '--assignee name' and fails with 'unknown flag'; pass the flags explicitly. Assignees must be repo collaborators: check with gh api repos/O/R/assignees/<login> (204 = assignable, 404 = not).