docs(mermaid): enforce actual line breaks for multiline text

The \n literal often fails to render correctly in Mermaid nodes. The
guidelines now require using actual newlines inside quoted text to
ensure multiline content displays reliably.

.ai/instructions/skills/mermaid.instructions.md

@@ -15,6 +15,20 @@ Use `-- "text" -->` syntax, **not** `-->|text|` syntax.
 
 ---
 
+## Line Breaks (Multiline Text)
+
+Do not use `\n` for line breaks inside Mermaid nodes or labels. It does not always render correctly.
+Instead, use actual line breaks (real newlines) inside quoted text.
+
+```
+✅ A["First line
+Second line"]
+
+❌ B["First line\nSecond line"]
+```
+
+---
+
 ## Color Contrast — CRITICAL
 
 All Mermaid diagrams MUST have sufficient color contrast. AI-generated diagrams often fail this.