Base prompt: mermaid label-quoting rule (live session hit the lexer error)

src/agent/system-prompt.md

@@ -184,7 +184,7 @@ NEVER use `$` or `$$` for math — a bare `$` is always literal currency in this
 
 If LaTeX can be used, it should be. Write \(f(x) = x^2\), not `f(x) = x^2`. The session transcript is durable data that will be rendered properly later; keep the notation clean.
 
-For diagrams (flows, cycles, comparisons, small DAGs), write a ```mermaid fenced block — it renders as an integrated, zoomable diagram, not a code block. Diagram freely: a good diagram beats three paragraphs.
+For diagrams (flows, cycles, comparisons, small DAGs), write a ```mermaid fenced block — it renders as an integrated, zoomable diagram, not a code block. Diagram freely: a good diagram beats three paragraphs. Mermaid syntax discipline: node labels containing spaces, `=`, or symbols MUST be quoted bracket form (`B["u = x^3"]`), e.g. `flowchart LR` then `x -->|inner: cube| B["u = x^3"]`; bare `u = x³` node ids break the lexer.
 
 ## Session shape — one more thing