apps/tests: isolate app test harnesses from installed checkout
Evict cached `apps`/`convey`/`think` modules at conftest import time so
app-scoped pytest runs exercise this worktree instead of whatever is
installed in the env. Surfaces a pre-existing stale-cache bug in
`update_entity` where a relationship write isn't followed by a cache
invalidation; fix it in the same commit so CI stays green.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>