Add copilot instructions (#10092)

Signed-off-by: Andrey Sobolev <haiodo@gmail.com>

.github/copilot-instructions.md

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+# Huly Platform - Copilot Instructions
+
+TypeScript/Svelte 4 monorepo using Rush.js (pnpm 10.15.1), Node >=20 <25, Webpack 5, Electron, Jest.
+
+## Code Style
+
+**TypeScript**: Strict types, interfaces over types, avoid `any`, export types separately
+**Svelte**: Script/style/markup order, reactive `$:`, stores for state, small focused components
+**Naming**: Files `kebab-case`, Components `PascalCase`, functions `camelCase`, constants `UPPER_SNAKE_CASE`
+
+## Structure
+
+- `models/*` - Shared types/models
+- `server-*` - Server packages
+- `plugins/*` - Client plugins
+- `packages/*` - Reusable utilities
+- Projects 2-3 levels deep, each with `package.json`
+
+## Rush Commands
+
+```bash
+rush install         # Install deps
+rush build           # Build all
+rush build --to PKG  # Build specific
+rush add -p PKG      # Add dependency
+```
+
+## Patterns
+
+- Always handle errors (proper Error subclasses, catch promises)
+- Use async/await, Promise.all() for parallel ops
+- Svelte stores for shared state, separate business logic
+- JSDoc public APIs, tests alongside code
+- Run `rushx test` before commit
+
+## Avoid
+
+❌ `any` without reason ❌ console.log() ❌ Mixed concerns ❌ Circular deps ❌ Ignoring TS errors
+
+## When Coding
+
+Infer location from context (models/server/plugins/packages), match existing patterns, include proper imports/types, add error handling, use existing utils first.