docs: Update copilot instructions to include development environment setup and quality checks

.github/copilot-instructions.md

@@ -64,17 +64,23 @@ def validate_path(path: Path, allowed_roots: List[Path]) -> bool:
 6. **Run quality checks**: `tox -e lint,type`
 7. **Update docs**: Add docstrings and update README if needed
 
+## Development Environment
+
+**Virtual Environment**: `.venv` (already created)
+- Activate: `.venv\Scripts\activate` (Windows) or `source .venv/bin/activate` (macOS/Linux)
+- All Python commands automatically use this environment through VS Code integration
+
 ## Common Commands
 
 ```bash
-# Setup
+# Setup (one-time)
 pip install -r requirements-dev.txt
 
-# Testing
+# Testing (uses .venv automatically)
 pytest tests -v
 pytest tests --cov=src/webdrop_bridge --cov-report=html
 
-# Quality
+# Quality checks
 tox -e lint      # Ruff + Black checks
 tox -e type      # mypy type checking
 tox -e format    # Auto-format code