fix(token-usage): attach the org-configured attribute map to events

The custom_attributes attribute (position 30) is the org-configured
map every event type carries — the checkpoint and session emitters
attach it, and the comment in emit_changed_buckets even reserved the
slot for it — but token-usage events never attached it, so
organization-specific reporting dimensions were missing from them.

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