[product-manager-impersonator] Additional-instructions starter snippets

## Feature Idea

**Summary:** Add a small copy-paste snippet library that helps new adopters customize their first agents with safe, repo-specific `additional-instructions` examples.

## Why a Customer Would Want This
Teams are told that `additional-instructions` is the primary customization mechanism, but first-time users still have to invent their own prompts from a blank slate. A starter library for common needs like repo conventions, ignored paths, label semantics, review tone, setup expectations, and persona focus would make the first customization safer and faster.

## Rough Implementation Sketch
- Add a "Customize your first agents" section to `docs/workflows/gh-agent-workflows.md` with 6-8 reusable `additional-instructions` snippets.
- Link the section from the getting-started flow in `docs/index.md` or the welcome blog post.
- Add short commented `with:` examples to key templates such as `gh-agent-workflows/pr-review/example.yml` and `gh-agent-workflows/product-manager-impersonator/example.yml`, following the existing pattern in `gh-agent-workflows/issue-triage/example.yml`.
- Include guidance to keep instructions specific, auditable, and repo-local.

## Why It Won't Be That Hard
This is mostly documentation and example-template work, with no runtime behavior changes. The standard input already exists, and `issue-triage` already demonstrates that commented `with:` examples fit the template style.

## Evidence
- `docs/blog/posts/welcome-to-the-factory.md:71-76` tells users to copy an `example.yml` and customize with `additional-instructions` / `setup-commands`, but does not provide starter snippets.
- `docs/workflows/gh-agent-workflows.md:151-159` documents `additional-instructions` only as a generic standard input.
- `docs/blog/posts/architecture-under-the-hood.md:71-77` explains that `additional-instructions` is powerful and gives narrative examples, but not copy-paste patterns.
- `gh-agent-workflows/pr-review/example.yml:13-19` and `gh-agent-workflows/product-manager-impersonator/example.yml:14-17` call the reusable workflows without commented customization examples.
- `gh-agent-workflows/issue-triage/example.yml:17-23` already uses a commented `with:` block to show label semantics, which is a good pattern to reuse.

## Suggested Actions
- [ ] Add starter `additional-instructions` snippets to the GH Agent Workflows docs.
- [ ] Link the snippet library from getting-started documentation.
- [ ] Add concise commented customization examples to 2-3 high-traffic workflow templates.

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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/27951043475)

Give us feedback! React with 🚀 if perfect, 👍 if helpful, 👎 if not.

Start with the commented with: example in gh-agent-workflows/issue-triage/example.yml, then compare docs/workflows/gh-agent-workflows.md, docs/index.md, and the referenced welcome post with the pr-review and product-manager-impersonator templates. Done means the documentation contains 6-8 reusable additional-instructions snippets, the getting-started flow links to them, and 2-3 high-traffic templates show concise commented customization examples.

Written by the indexing model from the issue text.