docs: update Quick start example to markdown prompt format

[Copilot]

The README Quick start showed a .ts file, but rig programs are typically authored as .md files with a nested ```rig ``` code block — the format used by all samples in skills/rig/samples/.

Changes

• README.md: Replace the ts fenced block with a md outer fence containing a # Review Git Diff heading and an inner ```rig ``` block, matching the canonical sample format
• Update the run command from program.ts → program.md

# Review Git Diff

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

const reviewDiff = agent({
  model: "small",
  instructions: p`Review ${p.bash("git diff -- .")} and return only the declared output.`,
  output: s.object({ summary: s.string, risk: s.enum("low", "medium", "high") }),
});

export default reviewDiff;
```