gl_install_pkg now returns GL_INSTALL_ERR_SCOPE for
scope == GL_SCOPE_USR && repo == "base", checked immediately after
manifest parse and before any store path construction or filesystem
mutation. Per SECURITY_DESIGN.txt section 8: enforced in the library
rather than a CLI, since the split-.so architecture lets third-party
tools call gl_install_pkg directly.
Also drops src/istoreutils.c.rej, .c.back, .h.back — stale leftovers
from the uid -> gl_context_t migration patch; that migration is fully
present in the current tree, so these were dead weight.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>