Export `tree.json` to Graphviz / Mermaid / interactive HTML

[skymanbp]

Idea

Every run already streams a structured tree.json. A small, dependency-light exporter would turn it into shareable visuals — which also doubles as marketing (people post the pictures).

Targets:

• Graphviz DOT — .dot for dot -Tsvg
• Mermaid — paste-into-Markdown flowchart
• Interactive HTML — a single self-contained file (collapsible radial/indented tree, verdict colours)

The existing tools/gen_radial_tree.py is a good reference for the radial layout + verdict glyphs; this issue is about driving a renderer from real tree.json instead of a hard-coded demo tree.

What's involved

A tools/export_tree.py reading tree.json and writing one of the formats. Start with DOT or Mermaid (smallest); HTML viewer is a nice stretch goal.