A user who knew another account's tree UUID could start a copilot
conversation, causing the tree's full node structure, names, and
descriptions to be sent to the AI as part of the system prompt.
Fix: add account_id (or is_default / visibility='public') filter to
the tree SELECT in copilot_service.start_conversation(). Returns 404
for inaccessible trees. Test added in test_tenant_isolation_p0.py.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>