chore: remove internal docs from repo, update public documentation

Remove docs/superpowers/, .claude/ config, and PRD.md from version
control (kept locally via .gitignore). Update README, CHANGELOG,
VitePress docs, and .env.example to reflect recent features: Files
page, teams, admin settings, persistent storage, and various API
improvements.

.gitignore

@@ -27,7 +27,9 @@ blob-report/
 
 # IDE / tool scratch
 .superpowers/
-.claude/settings.local.json
+.claude/
+docs/superpowers/
+PRD.md
 
 # Ad-hoc test screenshots and reports
 test-*.png