feat(design): write/read DESIGN.md in the open spec; persisted format choice

gstack's design skills now write DESIGN.md in the open DESIGN.md format and
read tokens from it. {{DESIGN_MD_CHECK}} renders the format check through
bin/gstack-design-md.ts: design-consultation's Phase 0 settles the format once
(spec → update tokens in the front matter; legacy without a marker → one
AskUserQuestion: convert with a .legacy.bak, keep the legacy file, or start
fresh; the answer is written into the file as the format marker so no skill
asks again; a marker already present is obeyed silently; unknown → prose;
missing → Phase 6 writes one). Phase 6's template is the spec form: YAML front
matter with name, description, and exactly the five token groups (colors,
typography.display/body/label/mono with fontFeature: tnum on mono, rounded,
spacing, components with {path} references), then Overview (Creative North
Star, product context, mode per surface, references, key characteristics),
Colors (opening with the Restrained / Committed / Full palette / Drenched
strategy), Typography, Layout, Elevation & Depth, Shapes, Components, Do's and
Don'ts, plus gstack's Motion and Decisions Log as extras; the template ends
with a check that the file parses as `spec`.

design-review runs the `:calibrate` form in Setup: a spec file's flat tokens
are the calibration source (a value present in the tokens is never a finding),
the marker is respected, and conversion is never offered there; its DESIGN.md
export writes the spec form. design-html's token extraction writes the spec
form and respects an existing choice. review/design-checklist.md category 5 and
ship's review-lite step 1 name `gstack-design-md tokens` as the calibration
source; plan-design-review Pass 5 cites tokens by path when front matter
exists.

The contract owns the bin's DESIGN_MD_MARKER / REASON / WRITTEN / BACKUP
lines; the contract test's pending list closes. Carve guard: design-
consultation skeleton 66,500 → 67,500 (measured 67,014; +1,508 B against the
1.5 KB cap). Codex and Factory ship goldens refreshed (review-lite step 1).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/design-detect-contract.test.ts

@@ -18,7 +18,8 @@ import { catalogEntry } from '../lib/design-catalog';
 const ROOT = path.join(import.meta.dir, '..');
 const TOKEN = /\b(IMPECCABLE_[A-Z_]+|DETECT_[A-Z_]+|DESIGN_MD_[A-Z_]+|DOM_DUMP_[A-Z_]+|DESIGN_DETECTOR_[A-Z_]+|DESIGN_DETECT_[A-Z_]+)\b/g;
 // Things that look like sentinels but are env vars / flags the prose legitimately names.
-const NOT_SENTINELS = new Set(['IMPECCABLE_BIN', 'IMPECCABLE_HOME', 'IMPECCABLE_HOOK_DISABLED', 'DESIGN_DETECT_TIMEOUT_MS']);
+// Env vars, flags, and resolver placeholder names the prose legitimately names.
+const NOT_SENTINELS = new Set(['IMPECCABLE_BIN', 'IMPECCABLE_HOME', 'IMPECCABLE_HOOK_DISABLED', 'DESIGN_DETECT_TIMEOUT_MS', 'DESIGN_MD_CHECK', 'DESIGN_DETECTOR']);
 
 function* agentReadableFiles(): Generator<string> {
   const skip = new Set(['node_modules', '.git', 'dist', 'build', 'test', 'docs', '.context', '.claude', '.agents', '.factory', '.cursor', '.kiro', '.opencode', '.openclaw', '.hermes', '.slate', '.gstack', '.gbrain', '.conductor']);
@@ -70,8 +71,7 @@ describe('contract shape', () => {
 });
 
 describe('every printable sentinel is mentioned somewhere the agent reads', () => {
-  // DESIGN_MD_* sentinels arrive with the DESIGN.md tool wiring; until then they are contract-only.
-  const PENDING = new Set<string>([SENTINEL.DESIGN_MD_FORMAT, SENTINEL.DESIGN_MD_CONVERT_REFUSED, SENTINEL.DESIGN_MD_INTERNAL_ERROR, SENTINEL.DESIGN_MD_TOKEN_REF_INVALID]);
+  const PENDING = new Set<string>();
   test('generated SKILL.md files, sections, or the checklist name each one', () => {
     const corpus = [...agentReadableFiles()].filter(f => !f.includes(`${path.sep}scripts${path.sep}`)).map(f => fs.readFileSync(f, 'utf-8')).join('\n');
     const selfDescribing = new Set(SELF_DESCRIBING_SENTINELS);