chore: Add AGENTS.md, symlink copilot instructions (#15145)

* chore: Add AGENTS.md, symlink copilot instructions

* clean up

Author: elliott-with-the-longest-name-on-github

.github/copilot-instructions.md

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+../AGENTS.md

.github/copilot-instructions.md

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+# SvelteKit Coding Agent Guide
+
+This guide is for AI coding agents working in the SvelteKit monorepo.
+
+**Important:** Read and follow `CONTRIBUTING.md` as well - it contains essential information about testing, code structure, and contribution guidelines that applies here.
+
+## Quick Reference
+
+### Essential Commands
+
+```bash
+# Initial setup (takes 3-4 minutes, set 10+ min timeout)
+pnpm install --frozen-lockfile
+
+# Build all packages (~1-2 seconds)
+pnpm build
+
+# Format code (~15 seconds)
+pnpm run format
+
+# Lint (takes 2-3 minutes, set 5+ min timeout)
+pnpm run lint
+
+# Type checking (takes 3-4 minutes, set 8+ min timeout)
+pnpm run check
+```
+
+### Testing Commands
+
+```bash
+# Unit tests only (fastest - ~6 seconds)
+pnpm -F @sveltejs/kit test:unit
+
+# Run a single unit test file
+pnpm -F @sveltejs/kit test:unit:dev path/to/test.spec.js
+
+# Integration tests (10-30 minutes, set 60+ min timeout)
+pnpm test:kit
+
+# A single integration test suite (name of suite found in packages/kit/test/apps/*/package.json)
+pnpm -F {name-of-suite} test
+
+# Run single Playwright test (must use workdir - no pnpm -F shorthand)
+cd packages/kit/test/apps/basics && npx playwright test --grep "test name"
+
+# Other package tests (5-15 minutes, set 30+ min timeout)
+pnpm test:others
+```
+
+### Pre-submission Checklist
+
+1. `pnpm run format` - Auto-format code
+2. `pnpm run lint` - Check code style (don't cancel early)
+3. `pnpm run check` - Type checking (don't cancel early)
+4. `pnpm -F @sveltejs/kit test:unit` - Run unit tests
+5. For @sveltejs/kit changes: `pnpm -F @sveltejs/kit prepublishOnly` - Generate types
+6. Run `pnpm changeset` to document changes (prefix with `fix`, `feat`, `breaking`, or `chore`)
+
+## Code Style Examples
+
+The coding style guidelines are in `CONTRIBUTING.md`. Here are additional examples:
+
+### Imports
+
+```javascript
+// JSDoc type imports at the top
+/** @import { Handle, RequestEvent } from '@sveltejs/kit' */
+
+// Named imports (no default exports)
+import { HttpError, SvelteKitError } from '@sveltejs/kit/internal';
+```
+
+### Functions
+
+```javascript
+// Exported named functions (no default exports)
+export function coalesce_to_error(err) {
+	// Implementation
+}
+
+// JSDoc for all parameters and return types
+/**
+ * @param {unknown} error
+ * @returns {Error}
+ */
+export function coalesce_to_error(error) {
+	// Implementation
+}
+
+// Use arrow functions for callbacks
+const handler = (event) => {
+	/* ... */
+};
+```
+
+### Error Handling
+
+```javascript
+// Type checking with instanceof
+if (error instanceof HttpError || error instanceof SvelteKitError) {
+	// Handle
+}
+
+// Graceful fallbacks
+const status = error?.status ?? 500;
+
+// Optional chaining and nullish coalescing
+const content_type = request.headers.get('content-type')?.split(';', 1)[0];
+```
+
+### TypeScript/JSDoc
+
+- Use JSDoc annotations for all function parameters and return types
+- Complex types: `/** @type {Array<{ type: string, subtype: string }>} */`
+- Type casting when needed: `/** @type {Error} */ (err)`
+- Enable strict mode: `checkJs: true`, `strict: true` in tsconfig.json
+
+### Formatting (via Prettier)
+
+- **Tabs for indentation** (not spaces)
+- **Single quotes** for strings
+- **No trailing commas**
+- **100 character line width**
+- Files are auto-formatted by `pnpm run format`
+
+### Comments
+
+````javascript
+// JSDoc with usage examples for public APIs
+/**
+ * Sequence multiple handle functions
+ *
+ * @example
+ * ```js
+ * export const handle = sequence(first, second);
+ * ```
+ *
+ * @param {...Handle} handlers
+ * @returns {Handle}
+ */
+
+// Inline comments for clarifications
+// no match equals invalid header — ignore
+````
+
+## Key Packages
+
+- `@sveltejs/kit` - Main framework (`packages/kit/`)
+- `adapter-*` - Platform adapters (node, cloudflare, netlify, vercel, static, auto)
+- `@sveltejs/package` - Package building utilities
+- `@sveltejs/enhanced-img` - Enhanced image component
+- `@sveltejs/amp` - AMP support
+
+## Troubleshooting
+
+- **Browser tests fail**: `pnpm playwright install chromium`
+- **Build failures**: Ensure `pnpm install --frozen-lockfile` completed
+- **Type errors**: Run `pnpm -F @sveltejs/kit prepublishOnly`
+- **Lint issues**: Run `pnpm run format` first

AGENTS.md

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+AGENTS.md