[product-manager-impersonator] Quick setup: validate `--workflows` IDs with suggestions

## Feature Idea

**Summary:** Add preflight validation for `--workflows` values in `scripts/quick-setup.sh` so unknown workflow IDs fail fast with clear guidance and nearest-match suggestions.

## Why a Customer Would Want This
New adopters frequently copy workflow IDs manually and a typo currently fails late during remote fetch with little context. A fast, explicit validation error would make first-run setup much smoother and reduce onboarding friction/support churn.

## Rough Implementation Sketch
- Build an allowlist of supported workflow IDs from the existing curated arrays in `scripts/quick-setup.sh`.
- Validate all CSV entries from `--workflows` before any `git fetch`/branch creation or remote `curl` loop.
- On invalid IDs, print a clear error that includes valid IDs and “did you mean …” suggestions for close matches.
- Document the behavior in quick setup docs where `--workflows` is described.

## Why It Won't Be That Hard
The script already centralizes workflow IDs and parses `--workflows`; this is a localized enhancement to argument validation and error messaging without changing workflow engine behavior or repo layout.

## Evidence
- `scripts/quick-setup.sh` parses `--workflows` CSV, but has no ID validation step before execution (`scripts/quick-setup.sh#L144-L149`).
- Unknown workflow names currently fail only when constructing/fetching a remote file URL (`scripts/quick-setup.sh#L194-L203`).
- Usage/docs ask users to provide comma-separated IDs manually, increasing typo risk (`scripts/quick-setup.sh#L25`, `gh-agent-workflows/README.md#L63-L65`).

> [!NOTE]
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> Integrity filtering activated and filtered the following items during workflow execution.
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> - issue:elastic/ai-github-actions#778 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
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> - issue:elastic/ai-github-actions#771 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#770 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#769 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
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> - issue:elastic/ai-github-actions#759 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#757 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
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[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/23484291199)

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