fix(baoyu-post-to-wechat): repair htmlToPlainText syntax error and entity decoding

Follow-up to the previous newspic conversion commit:

- `“` / `”` mapped to `"""` (three straight double-quotes),
  producing an unterminated string literal that prevented wechat-api.ts
  from parsing. Replaced with the intended curly Unicode characters.
- `‘` / `’` similarly mapped to straight `'`; switched to the
  curly singles to match the entity meaning.
- Numeric entities used `String.fromCharCode`, which mangles code points
  above 0xFFFF (e.g. emoji like `😀` → empty string). Switched to
  `String.fromCodePoint`.
- Added hexadecimal numeric entity support (`😀` etc.); the
  previous regex only matched decimal forms.
- Anchored the entity sub-patterns with `^...$` so the callback cannot
  accidentally re-match a substring of the captured entity.

skills/baoyu-post-to-wechat/scripts/wechat-api.ts

@@ -125,17 +125,20 @@ function htmlToPlainText(html: string): string {
     "—": "—",
     "–": "–",
     "…": "…",
-    "“": """,
-    "”": """,
-    "‘": "'",
-    "’": "'",
+    "“": "“",
+    "”": "”",
+    "‘": "‘",
+    "’": "’",
   };
-  text = text.replace(/&(?:[a-zA-Z]+|#\d+);/g, (entity) => {
+  text = text.replace(/&(?:[a-zA-Z]+|#x[0-9a-fA-F]+|#\d+);/g, (entity) => {
     if (entityMap[entity]) return entityMap[entity];
-    // 处理数字实体，如 {
-    const numMatch = entity.match(/&#(\d+);/);
+    const hexMatch = entity.match(/^&#x([0-9a-fA-F]+);$/);
+    if (hexMatch) {
+      return String.fromCodePoint(Number.parseInt(hexMatch[1]!, 16));
+    }
+    const numMatch = entity.match(/^&#(\d+);$/);
     if (numMatch) {
-      return String.fromCharCode(Number.parseInt(numMatch[1]!, 10));
+      return String.fromCodePoint(Number.parseInt(numMatch[1]!, 10));
     }
     return entity;
   });