config.py's parsers move to app/utils/env.py and are imported back; the
three incompatible truthiness variants collapse to one TRUTHY tuple.
Fixes the measured bug: SERVERKIT_ALLOW_PRIVATE_DOWNLOADS=on silently
read false in plugin_service (its tuple lacked 'on'); same for the pip
gate, SERVERKIT_STAGING, and SERVERKIT_DEMO_MODE. Ratchet: no new
os.environ truthiness tuples outside utils/env.py.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
