[product-manager-impersonator] Add quick-setup bundle consistency check

## Feature Idea

**Summary:** Add an automated check that keeps quick-setup workflow bundles in sync across script and docs so users always install what the docs promise.

## Why a Customer Would Want This
Teams adopt these workflows using docs-first onboarding, and they expect the `--continuous-improvement` set to be consistent everywhere. Drift between installation behavior and docs creates confusion, broken expectations, and extra support/debugging time during rollout.

## Rough Implementation Sketch
- Add a new script (or extend `scripts/check-nav-catalog.py`) to compare bundle lists in `scripts/quick-setup.sh` with `docs/workflows/gh-agent-workflows.md`.
- Fail CI when the two lists diverge, with actionable output showing missing/extra workflow slugs.
- Wire the check into `.github/workflows/ci.yml` alongside the existing nav/catalog consistency validation.
- Document the guard in `DEVELOPING.md` so contributors know how to run/fix it locally.

## Why It Won't Be That Hard
This is a small, additive guardrail: the lists are already explicit in source, there is already a pattern for docs consistency checks (`scripts/check-nav-catalog.py`), and CI already runs this style of validation. No runtime workflow behavior changes are required.

## Evidence
- `scripts/quick-setup.sh:128-142` includes `product-manager-impersonator` and `refactor-opportunist` in `continuous_improvement_workflows`.
- `gh-agent-workflows/README.md:67-80` documents both in the same `--continuous-improvement` set.
- `docs/workflows/gh-agent-workflows.md:28-30` omits those two workflows from the documented `--continuous-improvement` set.
- `scripts/check-nav-catalog.py:1-17,125-151` shows an existing CI-backed drift-check pattern that can be reused.
- `.github/workflows/ci.yml:27-33` currently runs nav/catalog consistency checks, but no quick-setup bundle consistency check.

## Duplicate Check
Checked open feature/PM issues and prior PM filings (including `#449`, `#523`, `#570`, `#619`, `#679`, `#709`, and `/tmp/previous-findings.json`); none request bundle consistency validation between quick-setup and docs.

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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/23189262239)

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