[Feature] Render Mermaid diagrams as inline Unicode art in chat (as pi already does)

[ruttybob]

Area: TUI / Coding agent and CLI

Problem

When the assistant generates a Mermaid diagram (flowchart, sequence, etc.), the raw fenced source is shown in a code block. Terminal users cannot visualize it without copying it out to a renderer, which breaks the flow of reading the response.

Proposed direction

Render Mermaid code blocks as inline Unicode box-drawing art directly in the chat.

The strongest argument for feasibility: pi already ships exactly this feature. Prime Agent's TUI is built on top of pi, and pi renders Mermaid via grok-mermaid in packages/coding-agent/src/modes/interactive/components/mermaid.ts — a markdown transformer that finds mermaid fenced blocks, renders them to styled Unicode art, and maps spans to theme colors. As a daily pi user I found this genuinely convenient and useful: diagrams become readable without leaving the terminal. Porting the same approach should be low-risk since it is proven in the upstream project.

Concretely:

1. Optional transform callback on the TUI Markdown component, applied after tab normalization and before markdown parsing
2. A mermaid-transformer module in packages/coding-agent using grok-mermaid
3. Wire into assistant text blocks and user messages (not thinking blocks)
4. Settings toggle for the rendering mode: streaming (default) / off (raw source)
5. Fall back to raw source when rendering fails or the diagram is wider than the terminal

Alternatives considered

• Render to an image and view externally — leaves the terminal, heavy for a chat flow
• Open in a browser-based renderer — same flow break

Additional context

I have a working implementation (transform hook + transformer module + settings toggle + tests) on a branch, modeled on pi's approach, and can share or rebase it if invited.

[ruttybob]

[snimu]

Addressed by #1893