chore: ignore the subagent-driven-development workspace

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -25,3 +25,6 @@ tsconfig.focus.json
 # "@wrnexus/*" bare specifiers (resolved via the root tsconfig.json `paths`,
 # which requires the scaffold to live inside the repo tree).
 **/test/.tmp-*/
+
+# Subagent-driven-development scratch (ledger, briefs, review packages)
+.superpowers/