Author spec.bs with metadata block and boilerplate omits

Part of #1006.

Create the top-level `spec.bs` with Bikeshed metadata block:

```

Title: Spectrum Design Data Specification (Bikeshed Prototype)

Shortname: spectrum-design-data

Level: 1

Status: LD

Group: Adobe Spectrum

URL: https://garthdb.github.io/design-data-spec/

Editor: Garth Braithwaite, Adobe https://www.adobe.com, garthdb@adobe.com

Abstract: Prototype rendering of the Spectrum Design Data spec using Bikeshed.

Markup Shorthands: markdown yes, css no

Boilerplate: omit conformance, omit copyright, omit feedback-header

`Status: LD` (Living Document) avoids W3C TR boilerplate. The `omit` directives strip W3C-process scaffolding that doesn't apply to an Adobe vendor spec.

Create the top-level spec.bs using the Bikeshed metadata block shown in the issue. Start by checking the repository root for existing specification or build conventions, then verify that the requested metadata is present and the conformance, copyright, and feedback-header boilerplate is omitted.

Written by the indexing model from the issue text.