[product-manager-impersonator] Additive workflow selection for quick setup

## Feature Idea

**Summary:** Add a `--add-workflows` option to `scripts/quick-setup.sh` so adopters can install the default core workflows plus a few named add-ons without replacing the core set.

## Why a Customer Would Want This
Teams trying AI GitHub Actions often want the recommended five-workflow starter set and one or two extra automations, such as `docs-patrol` or `dependency-review`. Today `--workflows` is documented as an override, so a command like `--workflows docs-patrol` installs only that add-on and silently omits the core review, triage, mention, and CI detective workflows. An additive flag would make the common "core plus one" path obvious and safer.

## Rough Implementation Sketch
- Add a new `--add-workflows CSV` option alongside `--workflows` in `scripts/quick-setup.sh`.
- Parse the CSV after initializing `workflows` from either the defaults or an explicit `--workflows` override, then append each requested workflow with the existing `append_workflow_if_missing` helper.
- Update the quick setup help text and docs to clarify that `--workflows` replaces the default set while `--add-workflows` keeps it.
- Add lightweight shell tests or script-level validation coverage for default-only, override-only, and default-plus-add-on behavior if the repo has an existing quick setup test pattern.

## Why It Won't Be That Hard
The script already has the necessary building blocks: the default workflow array, the override parsing path, and `append_workflow_if_missing` for de-duplicating workflows when `--continuous-improvement` adds extras. This is a small CLI parsing/docs change rather than a compiler or workflow-runtime change.

## Evidence
- `scripts/quick-setup.sh:21`-`27` lists the current quick setup options and has no additive workflow flag.
- `scripts/quick-setup.sh:115`-`121` defines the recommended default core workflows.
- `scripts/quick-setup.sh:138`-`143` shows that providing `--workflows` replaces the default list instead of extending it.
- `scripts/quick-setup.sh:145`-`159` already has `append_workflow_if_missing`, which can be reused for additive selection.
- `gh-agent-workflows/README.md:56`-`65` documents the default workflows and says `--workflows` overrides them.
- `docs/workflows/gh-agent-workflows.md:24`-`27` presents the core set and continuous-improvement add-ons as opinionated bundles, which this flag would make easier to compose.

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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/28584577132)

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Start in scripts/quick-setup.sh, reading the option help, default workflow initialization, override parsing, and append_workflow_if_missing sections cited in the issue. Then review the quick setup documentation in gh-agent-workflows/README.md and docs/workflows/gh-agent-workflows.md. Done means --workflows still replaces defaults, --add-workflows appends without duplicates, and help and docs explain both behaviors; add lightweight coverage if an existing shell-test pattern is found.

Written by the indexing model from the issue text.