chore: stop tracking docs/superpowers and ignore it

Remove the superpowers spec/plan scratch from the repo and add it to
.gitignore (also added to the global excludes) so it never gets tracked.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: meodai

.gitignore

@@ -1,3 +1,6 @@
 node_modules
 dist
 dist-demo
+
+# never track superpowers scratch
+docs/superpowers/