feat: add Copilot instructions for Chartifact

.github/copilot-instructions.md

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+# Copilot Instructions for Chartifact
+
+Welcome to the Chartifact codebase! This document provides essential guidance for AI coding agents to be productive in this repository. Chartifact is a low-code framework for creating interactive, data-driven documents such as dashboards, reports, and presentations. Below are the key aspects to understand:
+
+## Big Picture Architecture
+
+Chartifact consists of several interoperating modules:
+
+1. **Document Schema**: Defines plugins and components that communicate via reactive variables. Key components include:
+   - **Text**: Markdown with dynamic placeholders.
+   - **Inputs**: Textboxes, checkboxes, sliders, dropdowns.
+   - **Tables**: Sortable, selectable, and editable data grids.
+   - **Charts**: Vega and Vega-Lite visualizations.
+   - **Diagrams**: Mermaid diagrams, including data-driven generation.
+   - **Images**: Dynamic image URLs based on variables.
+   - **Presets**: Named sets of variable values for quick scenario switching.
+
+2. **Sandboxed Runtime**: Securely renders documents in isolated environments.
+
+3. **VS Code Extension**: Provides tools for editing, previewing, and exporting documents.
+
+4. **Web-Based Viewer and Editor**: Enables quick edits and sharing.
+
+5. **Export Tools**: Generate standalone HTML documents for sharing or embedding.
+
+## Developer Workflows
+
+### Build
+- For now, don't build. We can add this later.
+
+### Testing
+- Currently we don't have much test coverage. We can add this later.
+
+## Project-Specific Conventions
+
+1. **File Formats**:
+   - `.idoc.md`: Markdown format for human-readable documents.
+   - `.idoc.json`: JSON format for structured, programmatic generation. This is compiled to markdown.
+
+3. **Styling**:
+   - Use standard CSS for styling documents. Examples are provided for articles, dashboards, and slides.
+
+4. **Security**:
+   - Documents are rendered in sandboxed iframes.
+   - No custom JavaScript execution or raw HTML in Markdown.
+
+## Examples
+
+- Source code for examples is in the packages/web-deploy/json folder
+- Examples in this repo use the json format and are compiled to markdown.
+- If you are asked to create an example, do it in the packages/web-deploy/json folder, do not modify the destination folder docs/assets/examples - this will be populated by a build.
+- The examples are built manually for now.
+
+## Runtime host
+
+- There is a sandboxed runtime that securely renders documents available at https://microsoft.github.io/chartifact/view
+- The url above can accept a `load` parameter to specify the document to render, for example https://microsoft.github.io/chartifact/view/?load=https://raw.githubusercontent.com/microsoft/chartifact/562d086/packages/web-deploy/json/sales-dashboard.idoc.json
+- If you are asked to create an example, please provide a link in the PR comments so it can be previewed.