Exclude docs/superpowers/ planning artifacts from the release package

Specs and plans under docs/superpowers/ are agentic-workflow artifacts,
not plugin runtime files - keep them out of the shipped tarball the
same way tests/ already is.

tests/package_archive_test.sh

@@ -23,6 +23,9 @@ fi
 if printf '%s\n' "$LIST" | grep -Eq '^(\./)?tests/'; then
   fail "archive contains local test suite"
 fi
+if printf '%s\n' "$LIST" | grep -Eq '^(\./)?docs/superpowers/'; then
+  fail "archive contains internal docs/superpowers planning artifacts (specs/plans), not plugin runtime files"
+fi
 if printf '%s\n' "$LIST" | grep -Eq '(^|/)\._[^/]+$|(^|/)\.DS_Store$|__MACOSX'; then
   fail "archive contains macOS AppleDouble or Finder metadata"
 fi