Adds SendFile to the Slack platform, mirroring SendImage. Both use Slack's UploadFileV2Context, which already handles arbitrary file types. The engine routes attachments from `cc-connect send --file` (invoked by agents via the system-prompt-documented CLI) to this method via the core.FileSender interface assertion. https://claude.ai/code/session_01WyqXEst7Z8tiMbzeh4rTfX Co-authored-by: Claude <noreply@anthropic.com>