[product-manager-impersonator] Quick setup `--local-only` mode

## Feature Idea

**Summary:** Add a `--local-only` flag to `scripts/quick-setup.sh` so teams can generate and commit workflow files locally without auto-pushing and auto-opening a PR.

## Why a Customer Would Want This
Many first-time adopters want a safe, review-first onboarding path. Today quick setup is optimized for immediate automation, which can feel high-risk in repos with stricter release controls or internal review gates. A local-only mode would let teams inspect changes before publishing, while still benefiting from the same setup flow.

## Rough Implementation Sketch
- Add `--local-only` option parsing and help text in `scripts/quick-setup.sh`.
- Keep current behavior as default; when `--local-only` is set, skip `git push` and `gh pr create`.
- Print explicit next-step commands (`git push`, `gh pr create`) so users can continue manually.
- Document the new flag in `README.md` and `gh-agent-workflows/README.md` quick-setup sections.

## Why It Won't Be That Hard
This is a small, additive change in one script plus docs. The script already has clear flag parsing and centralized final publish steps, so this is a straightforward conditional around existing commands with no compiler/runtime changes.

## Evidence
- Auto-PR path is the documented default in the root README: `README.md` lines 30-39.
- Quick setup docs also state it creates a branch and opens a PR: `gh-agent-workflows/README.md` lines 57-59.
- Script currently supports `--dry-run` but no persistent local-only mode: `scripts/quick-setup.sh` lines 22-30.
- Script always pushes and creates a PR after commit: `scripts/quick-setup.sh` lines 270-272.
- Duplicate checks performed:
- Open PM ideas in `/tmp/previous-findings.json` and `repo:elastic/ai-github-actions is:issue in:title "[product-manager-impersonator]"` include quick-setup ideas for `--audit`, `--ref`, `--list-workflows`, auto-remote, and token prompt behavior, but not local-only/no-auto-PR.
- Additional search: `repo:elastic/ai-github-actions is:issue is:open ("quick-setup" OR "quick setup") ("local-only" OR "local only" OR "no pr" OR "skip pr" OR "dry run")` produced no matching local-only proposal.

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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/23046034243)

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