RenderMermaid is disabled by default and emits terminal text rather
than SVG/PNG; complex sequence diagrams with alt/else blocks become
hard to read at default widths. Add docs/render-mermaid.md covering
enablement (renderMermaid.enabled), config knobs (useAscii, paddingX,
paddingY, boxBorderPadding), output expectations, and limitations.
Link the new guide from the coding-agent README.
Fixes#961
