feat(config): CSS custom-properties scraper

[aram-devdocs]

Summary

Scan a list of CSS files for :root { --foo: 4px; } decls and
surface them as token candidates. Used by plumb init for
projects that track tokens in CSS.

Crate / area

plumb-config — figment loader + schemars schema emission

Phase / batch

Part of #20 → Batch 2C (Token adapters — DTCG + Tailwind + CSS custom properties).

Effort: S

PRD references

• PRD §12.3

Acceptance criteria

• Handles nested :root + media-query :root blocks.
• Units normalized to px where applicable.
• Integration test on fixture CSS.

Dependencies

• None (first in batch; batch-level deps gate this).

Review gates

• 02-spec-reviewer
• 03-code-quality-reviewer
• 05-architecture-validator
• 04-test-runner

How to deliver

Dispatch /gh-issue <this-issue-number>. See the root AGENTS.md and the scoped plumb-config/AGENTS.md for the invariants that apply.

Suggested delivery: gh-issue