docs: track superpowers working documents in git

The uups-bank-demo wave's SDD records (ledger, six task briefs and reports,
review diffs) and the brainstorm design mockups were git-ignored, so they
existed only on one sandbox VM and reached no remote — this repo had no remote
at all until now.

Removes `.superpowers/` from .gitignore and the `*` .gitignore the superpowers
plugin writes inside .superpowers/sdd/; the second blocks the directory even
with the first removed.

Excluded as ephemeral local-server state, and now ignored by name:
.last-port, .last-token (a 64-char session token for a brainstorm server on a
port that is long gone), and the per-session state/ directories.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Fnwzj6McD6kSkXwjUKFKxe

.gitignore

@@ -1,2 +1,6 @@
-.superpowers/
 .worktrees/
+
+# ephemeral brainstorm-server session state
+.superpowers/brainstorm/.last-port
+.superpowers/brainstorm/.last-token
+.superpowers/brainstorm/*/state/