flattenToMessage now places the typed text first and appends pasted-chip
content after it with a single leading space (file/line chips remain fenced
provenance blocks after that), instead of prepending all attachments. A
leading slash command therefore stays first and the paste reads as its
continuation — `/command <pasted>` rather than `<pasted>` then the command.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>