feat: add prologue diagram field at parity with hosted Stage

[dastratakos]

Summary

Reaches diagram parity with hosted Stage: the prologue can now carry an optional Mermaid diagram, rendered in the prologue sidebar with an inline SVG that expands into a zoom/pan dialog. The stage-chapters skill instructs the agent when and how to emit one.

Changes

• Added diagram: string | null to the shared PrologueSchema — flows through ingestion and the JSON prologue column with no migration.
• Vendored MermaidDiagram + a lazy-loaded mermaid-renderer (theme-aware, react-zoom-pan-pinch dialog, code-block fallback on render error); mermaid is code-split so it only loads when a diagram is viewed.
• Rendered the diagram in prologue-section.tsx and added a ## Diagram fenced block to the Markdown export.
• Documented the field (when to include, type guide, formatting rules) in skills/stage-chapters/SKILL.md.

Testing

pnpm typecheck, pnpm lint, pnpm test (337 passing), and pnpm build all pass locally; added schema, formatter, and storage round-trip test coverage for the new field.

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Summary by cubic

Adds an optional Mermaid diagram to the prologue for parity with hosted Stage. Diagrams render inline in the sidebar, open to a zoom/pan dialog, and are included in Markdown exports.

• New Features

  • Added diagram: string | null to PrologueSchema (defaults to null; flows through ingestion/storage; no migration).
  • Rendered via MermaidDiagram with lazy-loaded mermaid, theme-aware UI, zoom/pan dialog via react-zoom-pan-pinch, and code-block fallback on errors.
  • Markdown export adds a “Diagram” fenced mermaid block when present.
  • Updated skills/stage-chapters/SKILL.md with usage and formatting guidance.

• Bug Fixes

  • Markdown export now widens the mermaid code fence to exceed any backtick runs in diagram content, preventing fence breakouts.

^{Written for commit 293b239. Summary will update on new commits.}

[stage-review]

Ready to review this PR? Stage has broken it down into 8 individual chapters for you:

	Title
1	Add diagram field to PrologueSchema
2	Update CLI tests for new prologue schema
3	Implement lazy-loaded Mermaid rendering
4	Create MermaidDiagram component with zoom support
5	Integrate diagrams into the prologue UI
6	Export diagrams in prologue Markdown
7	Document diagram usage in skill instructions
8	Other changes

_{Chapters generated by Stage for commit 293b239 on Jun 8, 2026 5:57am UTC.}

[cubic-dev-ai]

1 issue found across 12 files

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[cursor]

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