[product-manager-impersonator] Quick setup resilient download retries

## Feature Idea

**Summary:** Add configurable retry/backoff for `quick-setup.sh` downloads so transient network failures don’t abort first-run setup.

## Why a Customer Would Want This
Teams adopt this project through a one-command installer, and setup currently fails hard on any transient `raw.githubusercontent.com` fetch error. That creates avoidable friction during onboarding and forces manual reruns.

## Rough Implementation Sketch
- Add `--download-retries` (default `3`) and `--download-retry-delay` (default `2`) flags to `scripts/quick-setup.sh`.
- Use those values in both download call sites (`trigger-*.yml` loop and `agentics-maintenance.yml`) with `curl --retry ... --retry-delay ... --retry-all-errors`.
- Preserve current fail-fast behavior after retries, but print a short summary of which workflow downloads still failed.
- Document the new flags in quick-setup docs so users can tune reliability for stricter CI networks.

## Why It Won't Be That Hard
This is a small, localized Bash change in one script plus docs updates. The script already centralizes all download operations in two places, so adding retry options is low-risk and does not require workflow schema/compiler changes.

## Evidence
- One-command onboarding is explicit in docs:
- `README.md:28-39`
- `gh-agent-workflows/README.md:44-55`
- Downloads are currently single-shot and fail the whole run (`set -euo pipefail`) with no retry path:
- `scripts/quick-setup.sh:9`
- `scripts/quick-setup.sh:194-203`
- `scripts/quick-setup.sh:206-213`
- Recent activity indicates setup UX is actively being improved (good timing for an incremental reliability win):
- PR `#991` (quick-setup messaging)
- PR `#1007` (setup PAT guidance)
- Issue `#1006` (setup docs/script guidance alignment)

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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/24884919108)

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