[product-manager-impersonator] Quick setup label bootstrap flag

## Feature Idea

**Summary:** Add an optional `--create-labels` mode to `scripts/quick-setup.sh` so first-time installs can bootstrap required repository labels via `agentics-maintenance`.

## Why a Customer Would Want This
Teams adopting these workflows expect quick setup to be “first-run ready.” Today, they still need a separate manual `create_labels` dispatch step, which is easy to miss and can cause avoidable workflow friction.

## Rough Implementation Sketch
- Add a `--create-labels` flag in `scripts/quick-setup.sh`.
- After workflow install (and branch/PR creation), print and optionally run a guided `gh workflow run agentics-maintenance.yml -f operation=create_labels` step.
- Preserve `--dry-run` behavior by showing the exact label-bootstrap command without executing it.
- Document the new option in setup docs alongside existing quick-setup flow.

## Why It Won't Be That Hard
The maintenance workflow already supports `create_labels`, and quick setup already installs `agentics-maintenance.yml` and orchestrates post-install steps. This is a small extension of existing script + docs behavior, not a new subsystem.

## Evidence
- `scripts/quick-setup.sh` installs `agentics-maintenance.yml` but has no label-bootstrap step (`scripts/quick-setup.sh:206-213`).
- The script finishes by commit/push/PR only (`scripts/quick-setup.sh:268-270`).
- Docs already say users should manually run `create_labels` after first install (`docs/workflows/gh-agent-workflows.md:157-163`).
- `agentics-maintenance` already exposes `create_labels` as a dispatch/call operation (`.github/workflows/agentics-maintenance.yml:38-54`, `.github/workflows/agentics-maintenance.yml:247-253`).

Related non-duplicate context: #965 addresses shared trigger defaults, but not automatic label bootstrapping from quick setup.

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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/24560353269)

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