[product-manager-impersonator] Make workflow model selection discoverable

## Feature Idea

**Summary:** Document the existing `model` workflow input and add commented copy-paste examples so repository maintainers can intentionally select an agent model without reading workflow source.

## Why a Customer Would Want This

Maintainers adopting the reusable workflows need to tune model choice for quality, speed, or cost, but the public documentation and starter triggers do not expose that capability. A maintainer should be able to discover the supported input from the workflow README and apply it directly in their repository, rather than reverse-engineering the `.md` source.

## Rough Implementation Sketch

- Add `model` and its default to the shared and per-workflow input tables, including `gh-agent-workflows/pr-review/README.md` and `gh-agent-workflows/product-manager-impersonator/README.md`.
- Add a commented `with.model` example to representative `example.yml` trigger files, with guidance to use a supported model identifier without maintaining a hardcoded allowlist.
- Link the examples from the GitHub Agent Workflows documentation and regenerate/validate only the normal authored workflow surfaces as appropriate.

## Why It Won't Be That Hard

The runtime wiring already exists: `.github/workflows/gh-aw-pr-review.md` passes `${{ inputs.model }}` into the engine. This is primarily a documentation and example-surface change, with no new runtime behavior or external service required.

## Evidence

- `.github/workflows/gh-aw-pr-review.md:19-32` already defines and wires the `model` input.
- `docs/workflows/gh-agent-workflows.md:151-159` documents the public inputs but omits `model`.
- `gh-agent-workflows/pr-review/README.md:25-34` and `gh-agent-workflows/product-manager-impersonator/README.md:28-38` omit the input from their per-workflow tables.
- `gh-agent-workflows/pr-review/example.yml:13-19` provides a copy-paste trigger without a discoverable model-selection hint.

Suggested actions:

- [ ] Document the existing `model` input and default in the public input tables.
- [ ] Add commented model-selection examples to workflow trigger templates.
- [ ] Add a short reference to supported model identifiers and validate the updated documentation/examples.

> [!WARNING]
>
> Firewall blocked 1 domain
>
> The following domain was blocked by the firewall during workflow execution:
>
> - `awmgmcpg`
>> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "awmgmcpg"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/30260371904)

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Start with .github/workflows/gh-aw-pr-review.md and docs/workflows/gh-agent-workflows.md to compare the existing model input wiring with the public input tables. Update the cited workflow READMEs and representative example.yml trigger files with the model input, default, and commented selection guidance. Done means maintainers can discover and copy the model option without reading workflow source, and the updated documentation and examples validate normally.

Written by the indexing model from the issue text.