Add /design round-trip and design-checker agent to the fleet

Design/branding for any user-facing repo becomes a vendor-neutral on-disk
contract — design/DESIGN.md (nine-section brief) + design/tokens.tokens.json
(W3C DTCG tokens) — that every agent reads before building UI. Claude Design
is the interchangeable cloud front-end, never a dependency.

- /design (main-thread command): inspiration-first scoping -> BRIEF.md -> user
  drives the cloud step -> distill the export back into the contract. Phase-C
  token distillation is agent-mediated because the export is inline-hardcoded
  HTML/CSS, not DTCG.
- design-checker (read-only subagent): audits a repo's UI against its own
  committed contract; says "run /design first" when none exists.
- /new-project scaffolds design/ for user-facing projects.

guides/new-project.md

@@ -134,6 +134,13 @@ Create `~/Projects/<name>/` and write, matching the standard exactly (`portabili
   from memory.
 - **`.claude/`** — create the directory; add `settings.json` only if a deterministic hook is
   wanted now. Don't add `rules/` symlinks until there's a `docs/guides/` file to point at.
+- **`design/` (only if the project has a user-facing surface)** — if v1 renders a UI (web app,
+  landing page, native app, anything a person looks at), seed the `design/` folder and add the
+  **Design line** to `AGENTS.md` ("before building or changing any user-facing UI, read
+  `design/DESIGN.md` and `design/tokens.tokens.json` and conform to them"). The contract and
+  folder layout are defined in `guides/design.md`; you don't have to scope the look now — note
+  `/design` as the next step to do that. Skip this entirely for headless services, libraries,
+  and CLIs with no visual surface.
 - **Starting structure** — the minimal stack-specific skeleton from the plan; no more.
 
 The stack's **quality gate** (linter + pre-commit hook) is deliberately *not* hand-rolled