Replaces the hand-written Dart snippets in `pointer_interceptor`'s README with `<?code-excerpt?>` pragmas backed by a new `example/lib/readme_excerpts.dart`, so the README's examples are validated against compilable, analyzed source (`update-excerpts --fail-on-change`) instead of free-hand text. One of the previous snippets was actually invalid Dart (a stray trailing comma), which this now catches. Also removes the package's `ci_config.yaml`, whose only content was `exempt_from_excerpts: true`, now that the excerpt validation actually runs and passes.

Part of flutter/flutter#102679.

## Pre-Review Checklist

## Test plan

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

**Note**: The Flutter team is currently trialing [Gemini Code Assist for GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code). Comments from `gemini-code-assist[bot]` aren't authoritative Flutter-team feedback; I'll wait for a human reviewer's guidance on which automated comments (if any) should be addressed.