Fix: unescape YAML quote escapes in DESIGN.md frontmatter scalars (#473)

* Fix: unescape YAML quote escapes in DESIGN.md frontmatter scalars (#428)

parseScalar() stripped a double-quoted scalar's outer quotes without
processing the backslash escapes inside, so a font stack that quotes a
multi-word family the CSS way, e.g.

  fontFamily: "\"IBM Plex Sans\", system-ui, sans-serif"

reached allowedFonts as '\"ibm plex sans' and design-system-font flagged
fonts DESIGN.md declares. Also collapses the doubled-quote escape in
single-quoted scalars and keeps a lone quote literal instead of slicing
it to an empty string. Applied to both copies of the parser
(cli/engine/design-system.mjs and skill/scripts/lib/design-parser.mjs).

Co-authored-by: Cursor Agent (AI-assisted change, reviewed and directed
by a maintainer)

* Decode YAML hex and Unicode escapes in double-quoted scalars

Review follow-up: the escape scanner only handled the simple set, so
\xNN, \uNNNN, and \UNNNNNNNN sequences stayed encoded and an escaped
token like "\x23b8422e" never matched #b8422e in CSS. Decode validated
hex escapes in both parser copies; malformed or out-of-range sequences
stay literal. Regression coverage for all three forms.

Co-authored-by: Cursor Agent (AI-assisted change, reviewed and directed
by a maintainer)

* Complete the YAML 1.2 double-quote escape set

Review follow-up: the escape map omitted the escaped space (\ ) and
non-breaking space (\_) forms, so fonts declared with them kept a
literal backslash in allowedFonts and their CSS declarations were
reported as undeclared. Map the full spec 5.7 set (\a \b \v \f \e
\N \L \P included) in both parser copies instead of chasing one escape
at a time. Regression coverage for both named forms.

Co-authored-by: Cursor Agent (AI-assisted change, reviewed and directed
by a maintainer)

cli/engine/design-system.mjs

@@ -142,10 +142,73 @@ function stripInlineYamlComment(s) {
   return s;
 }
 
+// YAML double-quoted scalars process backslash escapes. Stripping the outer
+// quotes without unescaping leaves them in place, so a nested font family like
+//   fontFamily: "\"IBM Plex Sans\", system-ui, sans-serif"
+// reaches allowedFonts as '\"ibm plex sans' and never matches the same family
+// declared in CSS. Scanner instead of a regex: the escape set is small and the
+// backslash handling stays readable.
+// The full YAML 1.2 double-quote escape set (spec section 5.7).
+const YAML_SIMPLE_ESCAPES = {
+  '0': '\0',
+  a: '\x07',
+  b: '\b',
+  t: '\t',
+  n: '\n',
+  v: '\v',
+  f: '\f',
+  r: '\r',
+  e: '\x1b',
+  ' ': ' ',
+  '"': '"',
+  '/': '/',
+  '\\': '\\',
+  N: '\u0085',
+  _: '\u00a0',
+  L: '\u2028',
+  P: '\u2029',
+};
+const YAML_HEX_ESCAPE_LENGTHS = { x: 2, u: 4, U: 8 };
+
+function unescapeYamlDoubleQuoted(body) {
+  let out = '';
+  for (let i = 0; i < body.length; i++) {
+    const ch = body[i];
+    if (ch !== '\\' || i === body.length - 1) {
+      out += ch;
+      continue;
+    }
+    const next = body[i + 1];
+    if (Object.prototype.hasOwnProperty.call(YAML_SIMPLE_ESCAPES, next)) {
+      out += YAML_SIMPLE_ESCAPES[next];
+      i++;
+      continue;
+    }
+    // \xNN, \uNNNN, \UNNNNNNNN. Malformed or out-of-range sequences stay
+    // literal rather than corrupting the rest of the scalar.
+    const hexLen = YAML_HEX_ESCAPE_LENGTHS[next];
+    if (hexLen) {
+      const hex = body.slice(i + 2, i + 2 + hexLen);
+      const codePoint = hex.length === hexLen && /^[0-9a-fA-F]+$/.test(hex) ? parseInt(hex, 16) : -1;
+      if (codePoint >= 0 && codePoint <= 0x10ffff) {
+        out += String.fromCodePoint(codePoint);
+        i += 1 + hexLen;
+        continue;
+      }
+    }
+    out += ch;
+  }
+  return out;
+}
+
 function parseScalar(raw) {
   const s = raw.trim();
-  if ((s.startsWith('"') && s.endsWith('"')) || (s.startsWith("'") && s.endsWith("'"))) {
-    return s.slice(1, -1);
+  if (s.length >= 2 && s.startsWith('"') && s.endsWith('"')) {
+    return unescapeYamlDoubleQuoted(s.slice(1, -1));
+  }
+  // Single-quoted YAML escapes only the quote itself, by doubling it.
+  if (s.length >= 2 && s.startsWith("'") && s.endsWith("'")) {
+    return s.slice(1, -1).split("''").join("'");
   }
   if (s === 'true') return true;
   if (s === 'false') return false;