Visual plan and diagram workflows

Provenance stamp: This issue was triaged on June 13, 2026 as part of the older-than-two-month GitHub issue triage exercise.

Source issue: #301

## Summary

Plannotator should track whether visual explanation and diagram workflows belong in the product, and if so, what the first useful version should be. This replaces the old standalone feature request with a more explicit product scope.

## What the source issue requested

- #301 requested diagram generation and re-iteration inspired by visual-explainer style tools. The user wanted Plannotator to generate diagrams and let the reviewer adjust or iterate on them as part of plan review.

## Triage determination

This issue did not have a strong collapse target in the first triage pass, which made it easy to lose. It deserves a dedicated current tracking issue because visual planning is distinct from text annotation, Ask AI, and code review.

## Acceptance criteria

- Decide whether diagrams are part of plan review, annotate mode, a separate visual-explainer skill, or a separate workflow.
- Define the first supported input: current plan, selected plan block, arbitrary markdown, or agent-generated prompt.
- Define the first supported output: Mermaid, SVG, HTML diagram, image, or another format.
- Support iteration on the generated diagram without forcing a full plan denial cycle.
- Keep diagram generation optional so normal plan review remains fast and focused.
- Close #301 after this replacement issue is created and the source request is represented here.