fix(usage): keep straddling and cost-only token buckets

Review fixes: a bucket overlapping the window start (since_ts inside
the 5-minute span) is now included instead of dropped; a bucket whose
latest revision has zero token counters but a positive transcript-
priced cost keeps its spend (only all-zero revisions count as
corrections); and the usage command's no-data check also considers
cost so spend-only activity is reported.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>