docs: group test-related sections under Testing in CONTRIBUTING.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: branchseer

CONTRIBUTING.md

@@ -36,7 +36,9 @@ just lint     # Clippy linting
 just doc      # Generate documentation
 ```
 
-### Running Specific Tests
+## Testing
+
+### Running Tests
 
 ```bash
 cargo test                                              # All tests
@@ -55,6 +57,17 @@ Integration tests (e2e, plan, fspy) require `pnpm install` in `packages/tools` f
 
 See individual crate READMEs for crate-specific testing details.
 
+### Playground
+
+The `playground/` directory is a small workspace for manually testing the task runner. It has three packages (`app → lib → utils`) with cached tasks (`build`, `test`, `lint`, `typecheck`) and an uncached `dev` script.
+
+```bash
+cargo run --bin vt -- run -r build          # run build across all packages
+cargo run --bin vt -- run -r --parallel dev  # start all dev scripts in parallel
+```
+
+See `playground/README.md` for the full task list and dependency structure.
+
 ## Cross-Platform Development
 
 This project must work on macOS, Linux, and Windows. Skipping tests on any platform is not acceptable.