Remove docs/superpowers and gitignore it

Planning artifacts (specs/plans) generated by Claude Code sessions don't belong
in the repo. Removing existing files and ignoring the directory going forward.

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

.gitignore

@@ -22,6 +22,7 @@ settings.local.json
 
 # Claude Code
 .claude/
+docs/superpowers/
 
 # Environment
 .env
@@ -32,3 +33,4 @@ docker-compose.override.yml
 # Misc
 *.log
 .DS_Store
+.superpowers/