Stop yarn enforcing the Node pin

yarn install refuses to run on a Node that does not match package.json
"engines", so pinning Node for Heroku broke CI (which runs 24.21.0) and would
block any contributor on another Node. Heroku still reads the pin; yarn no
longer enforces it.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.yarnrc

@@ -0,0 +1,4 @@
+# package.json pins one Node version in "engines" so Heroku builds fetch that
+# exact one. Don't make yarn refuse to install on any other Node: CI and
+# contributors run their own.
+ignore-engines true