Reconcile #5873's "timestamps in copied events" (merged to main) with
this branch's full-body copy rewrite: traceEventCopyText now prefixes
each line with the RFC 3339 timestamp when created_at is valid, on top of
the complete (untruncated) body, events separated by a blank line. The
two #5873 copy tests are updated to the full-body format; the orphaned
formatEventForClipboard helper is removed.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>