drift: continuous mandate enforcement for AI agents

A developer released drift, an open tool that continuously enforces AI agent payment mandates before each action. It closes a gap in Google's AP2 Python SDK, where the audience check is skipped for key-bound tokens, so a token minted for another audience is accepted.

• Six rules: amount, payee, audience, currency, window, intent
• Checked before the action, deny by default, frozen mandate
• MCP tools drift_check, drift_session, drift_explain
• Free 100 checks a day, 136 tests passing