Replaces the hand-written Dart snippets in `quick_actions`'s README with `<?code-excerpt?>` pragmas backed by `example/lib/readme_excerpts.dart`, so they're validated against compilable, analyzed source instead of free-hand text (the original used an untyped callback parameter and `print()`, which the repo's lint rules disallow). Also drops the package's `ci_config.yaml`, whose only purpose was opting it out of that validation.

Follows the same pattern as #12641 ([pointer_interceptor] Adopt code-excerpts for README).

Part of #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.