[product-manager-impersonator] Reuse existing COPILOT_GITHUB_TOKEN in quick setup reruns

## Feature Idea

**Summary:** Make `scripts/quick-setup.sh` reuse an already-configured `COPILOT_GITHUB_TOKEN` secret by default, and only prompt/write a new token when explicitly requested.

## Why a Customer Would Want This
Teams frequently rerun quick setup to add workflows or refresh triggers. Today reruns can still require token handling even when the repository already has `COPILOT_GITHUB_TOKEN`, which adds friction to an otherwise fast onboarding/update path.

## Rough Implementation Sketch
- Add a repo-secret existence check before the token prompt/write path (for example via `gh secret list --repo "$repo"`).
- If `COPILOT_GITHUB_TOKEN` already exists, skip prompt + `gh secret set` and print a clear "reusing existing secret" message.
- Add an opt-in flag (for example `--force-secret-update`) to intentionally rotate/reset the secret when desired.
- Keep existing `--skip-secret` and `--dry-run` behavior intact.

## Why It Won't Be That Hard
This is a small, isolated enhancement in one script (`scripts/quick-setup.sh`) with centralized option parsing and a single secret-management block, so the change is mostly a conditional branch plus a new flag and help text.

## Evidence
- `scripts/quick-setup.sh:L23-L30` documents existing flags; there is no secret-reuse or force-update mode.
- `scripts/quick-setup.sh:L215-L251` enters secret handling whenever `--skip-secret` is false, and writes the secret with `gh secret set`.
- `scripts/quick-setup.sh:L241-L243` errors in non-interactive mode when no env token is set, even though a repo secret may already exist.
- `README.md:L37-L39` positions quick setup as streamlined end-to-end install, so reducing rerun friction aligns with product expectations.
- Related but distinct existing ideas: `#809` (permission preflight), `#888` (existing-file overwrite policy), `#1220` (first-run checklist).

> [!NOTE]
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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/27343650420)

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