Add .github/copilot-instructions.md

[Copilot]

Generated repository-level Copilot instructions from codebase analysis.

Covers:

• Architecture — Two-thread Figma plugin model (worker code.ts ↔ UI main.js), message passing protocol, manifest structure
• Build system — Rollup dual-bundle config (IIFE for UI with HTML inlining, CJS for worker), dev/prod modes
• Source layout — All 14 source files in src/ with purpose annotations
• Code conventions — Svelte component ordering, naming (PascalCase components, camelCase vars, kebab-case CSS), 2-space indent, semicolons
• Key patterns — Selective update payloads, batch value distribution via fillToLengthOfSelected, Set deduplication for multi-select display, RGB↔HSL↔Hex color space conversions, async font loading with Promise.all
• Figma plugin specifics — figma global availability constraints, font loading requirements, missing weight handling