Add a bundled variables.css to the Tokens Studio release workflow

[pomfrida]

Part of #4740 (tooling track, alongside #5108).

Implement the bundle step decided in ADR-0010 (#5199): tokens_studio_release.yaml concatenates the generated CSS files in packages/eds-tokens/src/tokens/css/ and minifies the result into a committed src/tokens/css/variables.css.

Requirements

• Plain concat + minify — the bundle adds no behaviour on top of the generated files (colour-scheme behaviour is governed by ADR-0004; no prefers-color-scheme fallback, per ADR-0013 / PR docs: add ADR for attribute-scoped colour-scheme switching over light-dark() #5200)
• Glob the css/ directory rather than hard-coding the file list, so future export-set changes are followed automatically
• Reuse the assert-no-light-dark guard from 2.x on the bundled output as a regression check
• The committed file becomes importable on the beta line as @equinor/eds-tokens/next/css/variables.css through the existing ./next/css/* wildcard — no change needed in publish_tokens.yaml

Done when a beta release ships the bundle and it can be imported with one line.

[claude]

These issues look closely related and may be useful context:

• Validate the bundled CSS entry in beta and accept ADR-0010 before the flip #5202 — Validate the bundled CSS entry in beta and accept ADR-0010 before the flip (open): the direct downstream companion — it consumes next/css/variables.css from this issue, validates it on the beta line, and flips ./css/variables to the bundle once ADR-0010 is accepted.
• Publish the new tokens as a beta release line for @equinor/eds-tokens #5172 — Publish the new tokens as a beta release line for @equinor/eds-tokens (closed): established the @equinor/eds-tokens@beta line and the exports wildcard that makes the committed bundle importable — the mechanism this issue's "importable on the beta line" requirement relies on.
• Establish a new Token build & publishing pipeline #5108 — Establish a new Token build & publishing pipeline (closed): the parent tooling track this work sits alongside; it built the Tokens Studio CI/CD pipeline that tokens_studio_release.yaml extends here.

This comment was generated by an LLM and may be inaccurate. Please verify that the linked issues are actually relevant.

[pomfrida]

Implementation note: the requirement above says concat + minify, but the implementation in #5209 deliberately ships the bundle unminified (plain concatenation only). Rationale: the committed file stays a pure function of the source files (no minifier-version churn in release-PR diffs, no third-party minifier in the bundle step) and the release-PR diffs stay reviewable line by line — which matters since those PRs get no CI. The cost is ~0.2 kB gzipped (4.8 vs 4.6 kB), and consumers that bundle minify with their own tooling anyway.

ADR-0010 (#5199) has been updated to match. Everything else lands as specified: directory glob (no hard-coded file names), the parameterised assert-no-light-dark guard runs on the output, and the bundle is importable as @equinor/eds-tokens/next/css/variables.css through the existing ./next/css/* wildcard with no change to publish_tokens.yaml.