chore(audio): add download script, audio credits, and sprint 21/22 docs

- docs/download_audio.py: freesound batch downloader with all 22 confirmed IDs
  (API key removed — fill in locally from freesound.org)
- docs/credits-audio.md: generated CC-BY attribution table
- docs/superpowers/plans+specs: sprint 15, 21, 22 implementation plan/spec docs
- .claude/settings.json: enable experimental agent teams env var

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/settings.json

@@ -0,0 +1,5 @@
+{
+  "env": {
+    "CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS": "1"
+  }
+}