feat(toolchain): §25 Gitea @mention end-to-end integration

New files:
- src/api/mention_utils.py: extract_mentions(), infer_intent(),
  _build_response_guidance(), AGENT_ALIAS mapping
- templates/toolchain/mention.md: @mention notification template
  with context API + response guidance by intent type

Modified:
- src/api/toolchain_routes.py: S1-S5 handler changes
  - S1: Issue body @mention on opened
  - S2+S4: _handle_issue_comment control flow refactored
    (guard clause → positive if, CI + @mention independent paths)
  - S3: PR body @mention on opened
  - S5: Review body @mention on submitted
  - New _send_mention_mails() with auto-flow suppression
- src/daemon/toolchain_templates.py: register mention template

Design: docs/design/25-gitea-mention-toolchain.md (v2.0)
Tests: 405 passed, 3 skipped

src/daemon/toolchain_templates.py

@@ -24,6 +24,7 @@ _TEMPLATE_MAP: Dict[str, str] = {
     "review_updated": "review_updated.md",
     "review_comment": "review_comment.md",
     "review_merged": "review_merged.md",
+    "mention": "mention.md",
 }
 
 # 模板缓存