feat(live): v2 sidecar upgrade + preserve per-project config on build

Unify the design-system panel's data shape around DESIGN.md frontmatter
as the primary source of truth; the sidecar carries only what Stitch's
frontmatter schema can't (extensions + live component HTML + narrative).
Also fix a long-standing build bug that destroyed per-project config.

Shape changes:
- Server /design-system.json now returns { parsed, sidecar, hasMd,
  hasSidecar, mdNewerThanJson, parseError?, sidecarError? }. No more
  mode switching; both layers ship when present and the panel merges.
- Panel consolidates renderSidecarVisual + renderParsedMdVisual into a
  single renderDesignVisual that merges frontmatter primitives with
  sidecar extensions.colorMeta / typographyMeta. Helpers for color,
  typography, radii model-building. Parsed-md narrative synthesis
  survives as a fallback when no sidecar.
- DESIGN.json rewritten at schemaVersion 2: extensions.{colorMeta,
  typographyMeta, shadows, motion, breakpoints} + components (with
  refersTo pointing back to frontmatter component keys) + narrative.
  Token primitives no longer duplicated in the sidecar.

Build fix:
- scripts/build.js:634 wiped .claude/skills/ (and every other harness
  dir) on each rebuild, then recopied from dist. After commit b0feed0
  unbundled per-project config.json from dist, the sync destroyed the
  user's live-mode config on every build without replacing it.
- Added stashPerProjectArtifacts / restorePerProjectArtifacts in
  scripts/lib/utils.js. Hoisted PER_PROJECT_SCRIPT_ARTIFACTS to a
  module-level export so build.js and readSourceFiles share one
  source of truth. Build now preserves config.json across the sync.

Verified in browser: panel renders 10 colors, 9 typography roles, 3
shadows, 6 grouped components, 9 rules, 25 do/don't items, all merged
correctly from frontmatter + v2 sidecar with zero console errors.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/build.js

@@ -18,7 +18,7 @@
 import path from 'path';
 import fs from 'fs';
 import { fileURLToPath } from 'url';
-import { readSourceFiles, readPatterns } from './lib/utils.js';
+import { readSourceFiles, readPatterns, stashPerProjectArtifacts, restorePerProjectArtifacts } from './lib/utils.js';
 import { createTransformer, PROVIDERS } from './lib/transformers/index.js';
 import { createAllZips } from './lib/zip.js';
 import { generateSubPages } from './build-sub-pages.js';
@@ -631,8 +631,13 @@ async function build() {
     const skillsDest = path.join(ROOT_DIR, configDir, 'skills');
 
     if (fs.existsSync(skillsSrc)) {
+      // Preserve per-project script artifacts (e.g. live-mode config.json)
+      // across the rm + recopy. The build intentionally doesn't ship them,
+      // so without this the sync destroys local state on every rebuild.
+      const stashed = stashPerProjectArtifacts(skillsDest);
       if (fs.existsSync(skillsDest)) fs.rmSync(skillsDest, { recursive: true });
       copyDirSync(skillsSrc, skillsDest);
+      restorePerProjectArtifacts(skillsDest, stashed);
     }
   }