chore: add changeset for add-on dev watch mode

Author: tannerlinsley

.changeset/pretty-penguins-help.md

@@ -0,0 +1,10 @@
+---
+'@tanstack/cli': patch
+'@tanstack/create': patch
+---
+
+Add a continuous development workflow for custom add-on authors.
+
+- Add `tanstack add-on dev` to watch project files and continuously refresh `.add-on` outputs.
+- Rebuild `.add-on` assets and `add-on.json` automatically when source files change.
+- Document the new add-on development loop in the custom add-on guide.