Remove the temporary planning files in docs/superpowers

These design specs and implementation plans were working files for
developing the features they describe, written by an AI planning tool. They
were only ever meant to be local and are no longer needed: the features have
shipped, and the code, its comments and the README describe them.

The folder is now ignored, so such files stay local.

Author: lkiesow

.gitignore

@@ -27,3 +27,7 @@
 
 # Local Claude Code settings. Per-developer, not shared.
 .claude/
+
+# Specs and plans written by AI planning tools during development. Local
+# working files, not documentation.
+/docs/superpowers/