Add file attachment support to the support flow
Implements attachment capability across all layers of solstone's support
system: portal client method, CLI subcommand, convey workspace UI with
drag-and-drop, Flask route, and updated muse agent instructions.
Attachments are a follow-up action (post-creation) and go through the
same consent gate as all other outbound data — users review file names
and sizes before upload. Respects portal limits (10 MB/file, 5/upload).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>