[product-manager-impersonator] Quick setup PR should include a first-run checklist

## Feature Idea

**Summary:** Make `scripts/quick-setup.sh` create a tailored PR body (instead of `--fill`) that includes a first-run checklist with installed workflows and immediate next steps.

## Why a Customer Would Want This
Quick setup feels “done” after the PR opens, but new adopters still miss important post-install steps (especially label bootstrap). A built-in checklist in the generated PR makes first-run success much more reliable without requiring users to re-read docs.

## Rough Implementation Sketch
- Replace `gh pr create --fill` with explicit `--title` and generated `--body` in `scripts/quick-setup.sh`.
- Build checklist content from data the script already has (`workflows[@]`, `created_files`, flags like `--skip-secret`).
- Include concrete post-install actions in that body (for example, run `agentics-maintenance` with `operation=create_labels` when needed).
- Keep parity for `--dry-run` by printing the exact PR title/body that would be created.

## Why It Won't Be That Hard
All required inputs already exist in one place inside `quick-setup.sh`; this is mostly PR-body composition and minor docs updates, with no new subsystem or workflow architecture changes.

## Evidence
- `scripts/quick-setup.sh` already tracks installed artifacts (`created_files`) while downloading workflow triggers and maintenance workflow (`scripts/quick-setup.sh#L194-L213`).
- The current flow always opens PRs with generic autofill (`scripts/quick-setup.sh#L257` and `scripts/quick-setup.sh#L270`).
- Docs explicitly note a first-time post-install maintenance step (`create_labels`) that users must run manually (`docs/workflows/gh-agent-workflows.md#L157-L163`).
- Recent setup-focused activity suggests this path is active and worth smoothing further (for example PR #1190 and issue #974).

This proposal is distinct from existing open PM issues about label bootstrapping flags (#974), postflight file verification (#769), and shared defaults files (#965): this one targets PR-time onboarding clarity and first-run guidance.

> [!NOTE]
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> 🔒 Integrity filter blocked 40 items
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> The following items were blocked because they don't meet the GitHub integrity level.
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> - [#1175](https://github.com/elastic/ai-github-actions/pull/1175) `list_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#359](https://github.com/elastic/ai-github-actions/issues/359) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#1067](https://github.com/elastic/ai-github-actions/issues/1067) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#140](https://github.com/elastic/ai-github-actions/issues/140) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#424](https://github.com/elastic/ai-github-actions/issues/424) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - elastic/ai-github-actions#1067 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - elastic/ai-github-actions#1025 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#937](https://github.com/elastic/ai-github-actions/pull/937) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#505](https://github.com/elastic/ai-github-actions/issues/505) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#476](https://github.com/elastic/ai-github-actions/issues/476) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#704](https://github.com/elastic/ai-github-actions/issues/704) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#684](https://github.com/elastic/ai-github-actions/issues/684) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#875](https://github.com/elastic/ai-github-actions/issues/875) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#248](https://github.com/elastic/ai-github-actions/issues/248) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#245](https://github.com/elastic/ai-github-actions/issues/245) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#605](https://github.com/elastic/ai-github-actions/issues/605) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - ... and 24 more items
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> To allow these resources, lower `min-integrity` in your GitHub frontmatter:
>
> ```yaml
> tools:
> github:
> min-integrity: approved # merged | approved | unapproved | none
> ```
>
>

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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/26947720311)

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