fix: add MD031 compliance and division-by-zero protection

- Fix MD031 markdown linting in svg_flowchart_guidelines.md by adding
  blank lines around fenced code block
- Add NULLIF/COALESCE to postgre.md DISTINCT ON solution to guard
  against division by zero when first_orders is empty, returning 0.00
  instead of NULL or error

Author: myoshi2891

.agent/workflows/svg_flowchart_guidelines.md

@@ -17,6 +17,7 @@ description: SVG flowchart and data visualization best practices to avoid common
 1. **Always add 30-50px padding** below the last element in your viewBox height. If the last element ends at y=940 with ry=35, set viewBox height to at least `1000`.
 
 2. **For dynamic data tables**, calculate viewBox height based on number of rows:
+
     ```
     viewBox height = header_height + (row_count × row_height) + summary_text_spacing + padding
     ```

SQL/Leetcode/Intermediate Join/1174. Immediate Food Delivery II/Claude Sonnet 4.5 Extended/Immediate_Food_Delivery_II_postgre.md

@@ -86,9 +86,12 @@ WITH first_orders AS (
   ORDER BY customer_id, order_date
 )
 SELECT
-  ROUND(
-    100.0 * COUNT(*) FILTER (WHERE is_immediate) / COUNT(*),
-    2
+  COALESCE(
+    ROUND(
+      100.0 * COUNT(*) FILTER (WHERE is_immediate) / NULLIF(COUNT(*), 0),
+      2
+    ),
+    0.00
   ) AS immediate_percentage
 FROM first_orders;
 ```