fix(slack): deliver multi-file posts as one message with commentary (#657)

Slack uploads previously ran once per attachment with commentary posted
separately, splitting related files across messages. All non-empty files now
go out in a single upload call that carries the text as its comment, keeping
files above commentary; surfaces needing post options the upload call cannot
express fall back to the previous behavior. Returns an explicit uploaded flag
and the shared message timestamp for callers.

Co-authored-by: QM <noreply@ycombinator.com>