Add useplotly option with Plotly rendering support

[nmaguiar]

Motivation

• Add a new useplotly option so the agent can be primed to emit Plotly JSON fenced blocks (` ```plotly````) similar to existing Chart.js and Mermaid support.
• Render Plotly blocks in the browser UI only when the feature is enabled to avoid unconditional CDN loads.
• Provide guidance and consistent UX for interactive, multi-series visuals that benefit from Plotly's capabilities.

Description

• Wire a new boolean option through the stack by adding useplotly to mini-a.yaml, mini-a-web.yaml, mini-a-con.js, and CLI argument parsing in mini-a.js.
• Extend MiniA.buildVisualKnowledge in mini-a.js to include Plotly guidance and to preserve plotly code fences in final answers.
• Expose a /settings endpoint in mini-a-web.yaml and a small UI settings loader in public/index.md so the web UI can query whether Plotly is enabled.
• Implement client-side Plotly rendering in public/index.md including: conditional CDN loading (PLOTLY_CDN) only when useplotly=true, JSON parsing/validation, theming, container/CSS, destroy/resize/re-render helpers, and integration points to force rendering alongside existing Chart.js/Mermaid flows.
• Update documentation and examples (CHEATSHEET.md, USAGE.md, README.md) to document the new useplotly flag and example usage.

Testing

• Performed an automated headless UI smoke exercise that injected a plotly fenced block into the page and invoked the rendering routine via a headless browser script, which produced a screenshot artifact (artifacts/plotly-ui.png) indicating successful rendering.
• No unit test suites were modified or executed during this change; existing test jobs remain unchanged.
• The change was committed (git commit) after local verification of the UI rendering step.