feat: add comprehensive Copilot instructions for Kendo Themes repository

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file to onboard the Kendo Themes repository to GitHub Copilot coding agents. The instructions provide essential information to help coding agents work efficiently with this complex monorepo without extensive exploration.

What's Included

The instructions cover all critical aspects needed for successful development:

Repository Overview

• Clear description of the Lerna-managed monorepo structure with 7 theme packages
• Technology stack (SCSS/Sass, JavaScript, TypeScript, Lerna + Nx)
• Target runtime and project scope

Essential Build Commands

• Correct command sequencing to prevent common build failures
• Setup requirements (Node.js 22.x)
• Theme compilation (npm run sass)
• Documentation generation (npm run docs) - required for unit tests
• Development server startup (npm start)
• Linting for both scripts and styles

Critical Dependencies

• Unit tests require documentation to be generated first or they will fail
• Integration tests require distribution builds (npm run sass:dist)
• Git hooks validate changes on commit and push

Project Architecture

• Detailed file structure showing all 7 theme packages
• Component architecture (core → theme inheritance)
• SCSS compilation process with PostCSS autoprefixer
• Variable documentation patterns

CI/CD Pipeline

• Complete list of status checks that run on PRs
• Visual regression testing workflow
• Integration testing across multiple bundlers (webpack, vite, etc.)

Common Issues & Solutions

• Node version mismatches
• Build order dependencies
• Performance characteristics
• File exclusion patterns

Validation

All documented commands have been thoroughly tested:

• ✅ Build process from clean state
• ✅ Documentation generation
• ✅ Linting (scripts and styles)
• ✅ Development server startup
• ✅ Integration test dependencies
• ✅ Git hook validation

The instructions follow the 2-page limit while providing comprehensive coverage of the most important information coding agents need to avoid exploration overhead and build failures.

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[github-actions]

Packages Report

	core	default	classic	bootstrap	material	fluent	utils
Size	29.40 KB (0.0%)	763.69 KB (0.0%)	763.72 KB (0.0%)	776.95 KB (0.0%)	911.84 KB (0.0%)	1177.33 KB (0.0%)	625.19 KB (0.0%)
Gzip Size	4.85 KB (0.0%)	98.78 KB (0.0%)	99.12 KB (0.0%)	100.67 KB (0.0%)	111.72 KB (0.0%)	136.16 KB (0.0%)	52.16 KB (0.0%)
Compile Time	1 s (0.0%)	4 s (0.0%)	3 s (0.0%)	4 s (0.0%)	4 s (0.0%)	4 s (0.0%)	4 s (0.0%)