[product-manager-impersonator] Quick setup detective workflow mapping flag

## Feature Idea

**Summary:** Add a `--detective-workflows` option to `scripts/quick-setup.sh` so users can set PR Actions Detective’s watched CI workflow names during installation.

## Why a Customer Would Want This
Teams adopting these workflows expect the five-workflow quick start to work immediately. Today, PR Actions Detective only triggers when workflow names match the hard-coded defaults, so many repos will install successfully but never get CI failure diagnosis until they manually edit trigger YAML. A setup-time flag removes that hidden gotcha.

## Rough Implementation Sketch
- Extend `scripts/quick-setup.sh` argument parsing to accept `--detective-workflows "CI,Build,Test"` and validate non-empty CSV entries.
- After downloading `trigger-pr-actions-detective.yml`, rewrite `on.workflow_run.workflows` with the user-provided list when the flag is present.
- Document the new flag in `README.md` quick setup guidance and `gh-agent-workflows/README.md` quick setup section.
- Add a short cross-reference in `gh-agent-workflows/pr-actions-detective/README.md` so users can configure this without manual YAML edits.

## Why It Won't Be That Hard
This is a small, isolated enhancement in one script plus docs. The script already owns quick-setup download behavior, and the target config is a single known field in one generated trigger file (`workflow_run.workflows`). No compiler or lockfile format changes are required.

## Evidence
- `scripts/quick-setup.sh` lines 115-121: the default workflow set always includes `pr-actions-detective`.
- `gh-agent-workflows/pr-actions-detective/example.yml` lines 3-5: the trigger watches a fixed workflow-name list.
- `docs/workflows/gh-agent-workflows/pr-actions-detective.md` lines 23-25: docs explicitly tell users to manually update the `workflows` list to match repo CI names.
- `docs/index.md` lines 13-34: “Get Started in 60 Seconds” positions the starter set as immediately usable, including CI failure diagnosis.
- Recent activity trend is reliability-focused (for example PRs #1901 and #1905), so reducing first-run trigger misconfiguration fits current direction.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/32956906124)

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

Start with scripts/quick-setup.sh argument parsing and the downloaded trigger-pr-actions-detective.yml, then compare its workflow_run.workflows field with gh-agent-workflows/pr-actions-detective/example.yml. Update the quick setup guidance in README.md and gh-agent-workflows/README.md, add the cross-reference in gh-agent-workflows/pr-actions-detective/README.md, and verify that a sample CSV flag produces the requested workflow names.

Written by the indexing model from the issue text.