Fix quick-setup text drift and README license heading

> [!CAUTION]
> Protected files were modified in this change.
> This pull request is in `request_review` mode and requires explicit human scrutiny before merge.
>
> Protected files: `README.md`

## Summary

This resolves issue #1842 by applying the two text fixes requested by the text-auditor report.

1. Removed stale `--set-secret` from the top usage comment in `scripts/quick-setup.sh` so it matches the script's actual supported options.
2. Fixed the malformed Markdown in `README.md` by placing `## License` on its own heading line.

## Files changed

- `scripts/quick-setup.sh`
- `README.md`

Closes \#1842.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Text Auditor](https://github.com/elastic/ai-github-actions/actions/runs/32034344090)

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---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** pushSignedCommits: refusing unsigned push for branch 'fix-issue-1842-text-drift-1a4c91e9c61d7711': Signed-commit payload violates file-protection policy (request_review): README.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/32034344090)
>
> The bundle file is available in the `agent` artifact in the workflow run linked above.

To create a pull request with the changes:

```sh
# Download the artifact from the workflow run
gh run download 32034344090 -n agent -D /tmp/agent-32034344090

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-32034344090/aw-fix-issue-1842-text-drift.bundle refs/heads/fix-issue-1842-text-drift:refs/bundles/create-pr-fix-issue-1842-text-drift-1a4c91e9c61d7711-2ace7089
git update-ref refs/heads/fix-issue-1842-text-drift-1a4c91e9c61d7711 refs/bundles/create-pr-fix-issue-1842-text-drift-1a4c91e9c61d7711-2ace7089
git checkout fix-issue-1842-text-drift-1a4c91e9c61d7711
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-fix-issue-1842-text-drift-1a4c91e9c61d7711-2ace7089

# Push the branch to origin
git push origin fix-issue-1842-text-drift-1a4c91e9c61d7711

# Create the pull request
gh pr create --title 'Fix quick-setup text drift and README license heading' --base main --head fix-issue-1842-text-drift-1a4c91e9c61d7711 --repo elastic/ai-github-actions
```

Open scripts/quick-setup.sh and README.md and inspect the existing usage comment and License heading. Verify that the stale --set-secret text is removed and that ## License is on its own line; the matching two-file diff is the completion check.

Written by the indexing model from the issue text.