清除业务契约双读：抽奖/草稿/时间戳/邮件只认新格式，并删除 legacy 短代码改写。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/markdown/hide_test.go

@@ -177,20 +177,3 @@ func TestTimelineDoesNotBreakHideValidate(t *testing.T) {
 		t.Fatalf("timeline-only should pass: %v", err)
 	}
 }
-
-func TestRewriteLegacyDirectives(t *testing.T) {
-	src := "公开\n\n:::hide login\n秘\n:::\n\n:::timeline\n## 2026-01-01 A\n:::\n"
-	got := RewriteLegacyDirectives(src)
-	if strings.Contains(got, ":::") {
-		t.Fatalf("legacy remains: %q", got)
-	}
-	if !strings.Contains(got, "[hide login]") || !strings.Contains(got, "[/hide]") {
-		t.Fatalf("hide missing: %q", got)
-	}
-	if !strings.Contains(got, "[timeline]") || !strings.Contains(got, "[/timeline]") {
-		t.Fatalf("timeline missing: %q", got)
-	}
-	if err := ValidateHideContent(got); err != nil {
-		t.Fatal(err)
-	}
-}