chore: ignore local docs/superpowers planning docs (#4395)

Adds a gitignore rule for `**/docs/superpowers/` so locally-generated
planning and design scratch docs under that path aren't committed;
preventive only, no-op for existing tree.

Author: nicktrn

.gitignore

@@ -79,3 +79,6 @@ ailogger-output.log
 
 # local git worktree checkouts (not source) — keeps oxfmt/oxlint from descending into them
 .worktrees/
+
+# local planning/design docs, not committed
+**/docs/superpowers/