docs: drop explicit configureAgent(copilotEngine()) from README sample

[Copilot]

Copilot is now the default engine, so configureAgent(copilotEngine()) is boilerplate that shouldn't appear in the introductory example.

Changes

• Remove configureAgent(copilotEngine()) call and its blank line from the quick-start code block
• Drop configureAgent and copilotEngine from the import
• Update surrounding prose to no longer describe the engine configuration step

Before:

import { agent, configureAgent, copilotEngine, p, s } from "rig";

configureAgent(copilotEngine());

const reviewDiff = agent({ ... });

After:

import { agent, p, s } from "rig";

const reviewDiff = agent({ ... });