feat(pricing): default --start-date to today on schedule create

Summary

asc pricing schedule create is the top missing_required hotspot after the ones already fixed: 99 distinct installs (5.x, 14 days) omitted --start-date, and 46 hit 409 conflicts afterwards.

Proposed fix

• Default --start-date to today's date (UTC, the format Apple expects) when omitted; print the chosen date on stderr.
• Keep explicit values authoritative; reject malformed dates with exit 2 and the expected format.
• Note in help that Apple requires the date to be today or later.

Acceptance criteria

• cmdtest: omitted flag sends today's date and prints the note; explicit flag unchanged; malformed → exit 2
• help/docs updated