0.4.7: MCP daily buy budget, agent-first docs

New operator rail: PMQ_MCP_DAILY_USD caps cumulative buys per UTC day on top of the per-order cap (confirmed spend counts, rejections cost nothing, unknown outcomes consume conservatively until reconciled). The README now leads agent-first and documents the exact tool surface and every operator-set rail. Any MCP client works; the model cannot widen the rails from inside a session.