ci(docs): overlay design-data-spec/spec from beta when building docs

[GarthDB]

Description

Adds a step to the deploy-docs.yml workflow that overlays packages/design-data-spec/spec/ from origin/beta before the 11ty docs build runs. Only the spec markdown files are affected; everything else (site content, tokens, visualizers) continues to build from main.

Related Issue

N/A — temporary workflow shim; remove the overlay step once beta merges to main.

Motivation and Context

Spec proposals 010 (composite tokens, #817) and 012 (string-name escape hatch, #820) are merged on beta but beta is not yet ready to land on main. This lets the published spec at https://opensource.adobe.com/spectrum-design-data/spec/ reflect the latest proposals without a full beta merge.

The overlay is scoped to packages/design-data-spec/spec/*.md — the only files consumed by docs/site/scripts/copy-spec.js. Schema, conformance, fields, and rules changes on beta are not affected.

How Has This Been Tested?

Overlay verified locally:

git checkout origin/beta -- packages/design-data-spec/spec/
moon run site:copySpec
grep -c "composite\|string-name" docs/site/src/spec/token-format.md  # → 7
git checkout HEAD -- packages/design-data-spec/spec/

Both new sections ("Composite value types" and "String-name escape hatch") appeared in the rendered output.

Screenshots (if appropriate):

N/A

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2ff3f19

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[GarthDB]

Run report for 2ff3f19f

Total time: 7.3ms | Comparison time: 0s | Estimated loss: 7.3ms (100.0% slower)

	Action	Time	Status	Info
🟩	SyncWorkspace	7.2ms	Passed

Touched files

.github/workflows/deploy-docs.yml