[information-architecture] Quick setup docs advertise unsupported secret flags

## Information Architecture Findings

### 1. Quick setup documentation exposes flags the script rejects
**Area:** Action placement
**Component(s):** gh-agent-workflows/README.md, README.md, scripts/quick-setup.sh

**Problem:** The setup documentation presents secret-management flags as available controls, but the copy-paste setup script rejects them before setup begins. gh-agent-workflows/README.md lines 63-64 documents --skip-secret, and README.md lines 37-40 documents --set-secret as a supported or deprecated option. However, scripts/quick-setup.sh lines 5-8 omits both flags from usage and lines 32-63 treats unrecognized flags as fatal errors. Running bash scripts/quick-setup.sh --skip-secret exits with status 1 and prints Unknown flag: --skip-secret.

Users following either documented setup path cannot reach workflow installation because the advertised control is not part of the actual script interface. This is a concrete onboarding dead end, distinct from merely hiding required secrets or permissions.

**Suggested improvement:** Remove --skip-secret and --set-secret from all documentation and state the current built-in-token behavior consistently, or implement one supported flag and keep its help text, README guidance, and script behavior synchronized.

## Suggested Actions
- [ ] Align gh-agent-workflows/README.md and README.md with the flags accepted by scripts/quick-setup.sh.
- [ ] Add a regression check that every documented quick-setup option is accepted by the script.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Information Architecture](https://github.com/elastic/ai-github-actions/actions/runs/29111540768)

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

Start with the documented options in gh-agent-workflows/README.md and README.md, then compare them with the accepted flags and unknown-flag handling in scripts/quick-setup.sh. Run the script with the documented options, including --skip-secret and --set-secret, and add the regression check described in the issue. Done means the documentation and script interface agree, with no advertised option rejected.

Written by the indexing model from the issue text.