chore(repo): untrack docs/superpowers + ignore AI tool artefacts

Past Claude / Superpowers session output (plans + design specs) was
landing in the public repo. Files stay on disk locally but are no
longer tracked. .gitignore extended to cover the usual AI-tooling
scratch directories (.claude/, .cursor/, .codex/, .aider*) so future
sessions don't sneak anything in either.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -39,3 +39,10 @@ next-env.d.ts
 
 # prisma generated
 /src/generated/prisma
+
+# AI session artefacts (kept locally, not committed)
+/docs/superpowers/
+.claude/
+.cursor/
+.codex/
+.aider*