feat: add --page convenience option for trace list CLI

Found during v0.5.20 release review. Deferred because: new feature, not a fix.

The pagination UX requires users to manually calculate offsets: `--offset 20 --limit 20`, then `--offset 40 --limit 20`. A `--page N` option (where page 2 = offset 20 with default limit 20) would be more natural for the target audience. The footer already prints the exact next command, which mitigates this.

Flagged by: End User.