Scaffold plan & learn modes, project instructions (#251595)

* Scaffold plan & learn modes, project instructions

* Add the .vscode folder as a source for instructions

.vscode/project.instructions.md

@@ -0,0 +1,61 @@
+---
+applyTo: '**'
+---
+
+# VS Code Copilot Development Guide
+
+This file contains key information to help AI assistants work more efficiently with the VS Code codebase.
+
+## Quick Reference for Common Issues
+
+### Build & Test Workflow
+1. **Compile**: `npm run compile` (required before testing code changes)
+2. **Run specific tests**: `./scripts/test.sh --grep "pattern"`
+3. **Test file location**: `out/` directory contains compiled JavaScript
+4. **Extension compilation**: Extensions compile separately and take significant time
+
+### Code Architecture Patterns
+
+#### Testing Strategy
+- Unit tests in `src/vs/*/test/` directories
+- Integration tests in `test/` directory
+- Use `npm run compile` before running node-based tests
+
+## Common Gotchas
+
+### Module Loading
+- Use compiled files from `out/` directory when testing with node
+- Import paths: `const { Class } = require('../out/vs/path/to/module.js')`
+- ES modules require `.mjs` extension or package.json type modification
+
+### Test Location
+- Don't add tests to the wrong test suite (e.g., adding to end of file instead of inside relevant suite)
+- Look for existing test patterns before creating new structures
+- Use `describe` and `test` consistently with existing patterns
+
+## Investigation Shortcuts
+
+### Finding Related Code
+1. **Semantic search first**: Use file search for general concepts
+2. **Grep for exact strings**: Use grep for error messages or specific function names
+3. **Follow imports**: Check what files import the problematic module
+4. **Check test files**: Often reveal usage patterns and expected behavior
+
+### Build Optimization
+- Compilation takes ~2 minutes - do this once at start
+- Extensions compile separately - skip if not needed
+- Use incremental compilation for faster iteration
+
+## File Structure Quick Reference
+
+```
+src/vs/
+├── base/common/           # Core utilities (color.ts, etc.)
+├── editor/contrib/        # Editor features
+├── platform/             # Platform services
+└── workbench/            # Main UI components
+
+test/                     # Integration tests
+out/                     # Compiled output
+scripts/                 # Build and test scripts
+```