Fix quick-setup docs for unsupported flags

> [!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
- remove unsupported quick-setup flag references from contributor docs
- align `scripts/quick-setup.sh` header usage example with the script's actual supported options
- keep quick-setup guidance focused on implemented flags (`--workflows`, `--continuous-improvement`, `--repo`, `--branch`, `--dry-run`)

## Files changed
- `README.md`
- `gh-agent-workflows/README.md`
- `scripts/quick-setup.sh`

Closes \#2080

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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Newbie Contributor Patrol](https://github.com/elastic/ai-github-actions/actions/runs/34837041064)

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.

---

> [!NOTE]
> This was originally intended as a pull request, but the git push operation failed.
>
> **Original error:** ERR_VALIDATION: pushSignedCommits: refusing unsigned push for branch 'fix-issue-2080-quick-setup-flags-c442a0fe508ce077': Signed-commit payload violates file-protection policy (request_review): README.md, gh-agent-workflows/README.md
>
> **Workflow Run:** [View run details and download bundle artifact](https://github.com/elastic/ai-github-actions/actions/runs/34837041064)
>
> 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 34837041064 -n agent -D /tmp/agent-34837041064

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-34837041064/aw-elastic-ai-github-actions-fix-issue-2080-quick-setup-flags.bundle refs/heads/fix-issue-2080-quick-setup-flags:refs/bundles/create-pr-fix-issue-2080-quick-setup-flags-c442a0fe508ce077-44ae83c5
git update-ref refs/heads/fix-issue-2080-quick-setup-flags-c442a0fe508ce077 refs/bundles/create-pr-fix-issue-2080-quick-setup-flags-c442a0fe508ce077-44ae83c5
git checkout fix-issue-2080-quick-setup-flags-c442a0fe508ce077
# 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-2080-quick-setup-flags-c442a0fe508ce077-44ae83c5

# Push the branch to origin
git push origin fix-issue-2080-quick-setup-flags-c442a0fe508ce077

# Create the pull request
gh pr create --title 'Fix quick-setup docs for unsupported flags' --base main --head fix-issue-2080-quick-setup-flags-c442a0fe508ce077 --repo elastic/ai-github-actions
```

Review README.md, gh-agent-workflows/README.md, and the usage header in scripts/quick-setup.sh. Compare the documented options with the script's implemented flags, then verify that the quick-setup guidance consistently lists only supported flags and that the documentation remains aligned with the script.

Written by the indexing model from the issue text.