[product-manager-impersonator] Quick setup legacy secret flag compatibility

## Feature Idea

**Summary:** Let `scripts/quick-setup.sh` accept legacy secret-related flags such as `--set-secret` and `--skip-secret` as deprecated no-ops with a clear warning that PAT/COPILOT secrets are no longer required.

## Why a Customer Would Want This
Teams adopting the new no-PAT setup may reuse an older install command, an internal runbook, or the current docs and hit an immediate `Unknown flag` failure before any workflows are installed. A compatibility shim keeps the onboarding path smooth while teaching users the new `GITHUB_TOKEN`-based behavior.

## Rough Implementation Sketch
- Add parser cases in `scripts/quick-setup.sh` for `--set-secret` and `--skip-secret` that print a deprecation warning and continue.
- Update the usage text and quick setup docs so they say the flags are accepted only for backward compatibility.
- Add a small shell smoke test, if the existing test harness has one, to confirm both flags no longer abort argument parsing.

## Why It Won't Be That Hard
The surface area is tiny: `quick-setup.sh` already centralizes argument parsing in one `case` statement, and no secret-writing behavior needs to be restored now that workflows authenticate through `GITHUB_TOKEN`.

## Evidence
- `README.md:37-40` says quick setup no longer needs a personal access token, but still references `--set-secret` as deprecated.
- `gh-agent-workflows/README.md:63-65` still documents `--skip-secret` for manual secret handling.
- `scripts/quick-setup.sh:4-7` still includes `--set-secret` in the usage comment, while `scripts/quick-setup.sh:32-63` has no parser case for `--set-secret` or `--skip-secret`, so either flag falls into `Unknown flag`.

Duplicate check: existing product-manager issues cover token reuse, permission preflights, first-run checklists, label bootstrap, and other quick-setup improvements, but I found no existing issue specifically for accepting the deprecated secret flags as backward-compatible no-ops.

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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/27823111764)

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

Start with argument parsing in scripts/quick-setup.sh:4-7 and 32-63, then review README.md:37-40 and gh-agent-workflows/README.md:63-65. Confirm whether an existing shell smoke-test harness covers quick setup. Done means both legacy flags continue without an unknown-flag failure, emit a clear deprecation warning, and the referenced documentation describes their compatibility-only behavior.

Written by the indexing model from the issue text.