chore(gitignore): ignore docs/superpowers/

Local agent-generated design specs land under
``docs/superpowers/`` and shouldn't slip into PRs by default.
``git add`` will now refuse new paths there without ``--force``.
The three already-tracked specs in that tree stay tracked
(``.gitignore`` only filters untracked paths).

Author: tfcollins

.gitignore

@@ -140,3 +140,8 @@ lg_*
 
 # One-off local operator helper, never tracked.
 /setup-direct-mode.sh
+
+# Local superpowers/agent-generated design specs — keep out of PRs by
+# default.  Already-tracked files in this tree continue to be tracked
+# (.gitignore only filters untracked paths).
+docs/superpowers/