* feat(coding-agent): render Mermaid code blocks as inline Unicode diagrams Ports pi-mono's Mermaid rendering (grok-mermaid) into prime-agent: - pi-tui Markdown gains an options.transform hook that rewrites source Markdown with the exact content width before parsing (ported from upstream pi-tui). - New mermaid.ts transform replaces top-level mermaid code blocks with themed Unicode diagrams, falls back to the raw block when the diagram is wider than the message, and appends a themed warning line for partially rendered final diagrams. - AssistantMessageComponent applies the transform to text blocks only (never thinking blocks) and tracks streaming state so the streaming-to-final transition re-renders. - New markdown.mermaid setting (off/final/streaming, default streaming) with a settings-selector entry. Linear: ENG-5673 * test(coding-agent): kill mermaid mutation survivors Adversarial review mutation testing found three surviving mutants: - width guard flipped to >= (exact-fit diagrams would fall back) - codeSpan fence reduced to a single backtick (backtick labels break) - isStreaming inverted at the message_end call site (final mode never renders) Adds an exact-width boundary test, a backtick-label test, and a handleEvent-level final-mode transition test that kill them. * build(coding-agent): pin grok-mermaid exactly for renderer-output stability * test(coding-agent): collapse redundant mermaid coverage * docs(coding-agent): tighten mermaid comments * fix(coding-agent): tolerate non-object markdown settings values Normalize a malformed markdown settings value at the migration boundary (mirroring the telemetry handling) so changing the Mermaid mode cannot throw, and add the missing tui changelog fragment. * test(coding-agent): drop mermaid.test.ts * chore(coding-agent): one-line mermaid codeSpan comments