[product-manager-impersonator] Quick setup postflight verifier

## Feature Idea

**Summary:** Add a `--verify-install` postflight validation mode to `scripts/quick-setup.sh` that checks generated workflow files before commit/push.

## Why a Customer Would Want This
New adopters expect the “60-second setup” flow to produce a working configuration on the first try. Today, quick setup downloads files and proceeds directly to commit/push, so malformed or partial trigger files are often discovered only later in CI. A built-in postflight verifier would provide immediate, actionable feedback and reduce onboarding friction.

## Rough Implementation Sketch
- Add `--verify-install` (default on, optional `--no-verify-install`) in `scripts/quick-setup.sh` option parsing.
- After downloads, validate each generated `trigger-*.yml` is non-empty and contains a reusable workflow reference pattern (e.g., `uses: elastic/ai-github-actions/.github/workflows/gh-aw-*.lock.yml@v0`).
- Validate expected artifacts from `created_files` (including `agentics-maintenance.yml`) before `git add` / `git commit`.
- Document behavior and flags in `gh-agent-workflows/README.md` and `docs/workflows/gh-agent-workflows.md`.

## Why It Won't Be That Hard
This is a small, isolated enhancement in an existing script with centralized option parsing and clear install stages. The script already tracks generated files (`created_files`) and has a single commit/push path, so adding a validation gate is straightforward. There is prior repository precedent for install verification checks in CI, which can be mirrored for local postflight checks.

## Evidence
- `scripts/quick-setup.sh:194-203` downloads trigger workflows and appends to `created_files`.
- `scripts/quick-setup.sh:206-213` downloads `agentics-maintenance.yml` into the same install flow.
- `scripts/quick-setup.sh:268-270` commits/pushes immediately after staging, with no post-download validation step.
- `gh-agent-workflows/README.md:53-55` describes quick setup as an end-to-end install flow (including PR creation), increasing value of immediate verification.
- `docs/index.md:37-44` sets “commit and push, that’s it” expectations for first-run success.
- `.github/workflows/smoke-test-install.yml:103-129` already implements artifact-count/install verification patterns, showing a reusable model for validation logic.

> [!NOTE]
>
> 🔒 Integrity filtering filtered 43 items
>
> Integrity filtering activated and filtered the following items during workflow execution.
> This happens when a tool call accesses a resource that does not meet the required integrity or secrecy level of the workflow.
>
> - issue:elastic/ai-github-actions#765 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#768 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#766 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#759 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#753 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#757 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#756 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#752 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#749 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#748 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#747 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#746 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#745 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#744 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#733 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - issue:elastic/ai-github-actions#731 (`list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".)
> - ... and 27 more items
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>

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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/23432422115)

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