fix(mention): address PR #45 review feedback (M1-M3, S1-S3)

M1: Remove '帮忙' from help_keywords to fix keyword priority bug
M3: Add unit tests for mention_utils (§25.7)
S1: Merge two 'if action == opened' blocks in _handle_issues
S2: Extract _send_review_mentions helper to deduplicate @mention code
S3: Remove redundant 'import re' inside conditional block

src/api/mention_utils.py

@@ -110,8 +110,8 @@ def infer_intent(body: str) -> str:
     if any(kw in body for kw in assign_keywords):
         return "assign"
 
-    # 求助关键词
-    help_keywords = ["怎么", "如何", "？", "?", "什么", "哪个", "能否", "帮忙"]
+    # 求助关键词（注意："帮忙"已由 assign_keywords 的"帮忙做"覆盖，"请帮忙"由 collab_keywords 覆盖）
+    help_keywords = ["怎么", "如何", "？", "?", "什么", "哪个", "能否"]
     if any(kw in body for kw in help_keywords):
         return "help"