[product-manager-impersonator] Starter prompt cookbook for mention workflows

## Feature Idea

**Summary:** Add a “Try it now” starter prompt cookbook to `mention-in-issue` and `mention-in-pr` docs/examples so users know exactly what to ask right after installation.

## Why a Customer Would Want This
Teams can install mention workflows quickly, but first-run activation still depends on inventing a good `/ai` prompt. New adopters frequently stall at this step; concrete starter prompts lower time-to-value and make adoption less trial-and-error.

## Rough Implementation Sketch
- Add a **Try it now** section to `gh-agent-workflows/mention-in-issue/README.md` and `gh-agent-workflows/mention-in-pr/README.md` with 4–6 copy/paste prompts.
- Add brief commented starter prompt examples near the `/ai` trigger in both `example.yml` files.
- Reuse the existing documentation pattern used in `dependency-review` (“Manual usage with mention-in-pr”) to keep style consistent.
- Keep examples focused on common intents (triage, root-cause hypothesis, patch proposal, test plan).

## Why It Won't Be That Hard
This is a small documentation/example update across a handful of files with no workflow engine changes. The repo already has a proven “manual usage prompt” pattern in `dependency-review`, so this is mostly replication and tailoring for mention workflows.

## Evidence
- `README.md:25` lists `mention-in-issue` and `mention-in-pr` in the top starter workflow set, so onboarding quality here has outsized impact.
- `gh-agent-workflows/mention-in-issue/example.yml:17-19` and `gh-agent-workflows/mention-in-pr/example.yml:19-20` enforce `/ai` prefix triggers but provide no starter prompt guidance.
- `gh-agent-workflows/mention-in-issue/README.md:7` and `gh-agent-workflows/mention-in-pr/README.md:7` explain activation but not concrete first prompts.
- `gh-agent-workflows/dependency-review/README.md:46-52` already demonstrates a “Manual usage with mention-in-pr” prompt pattern that can be reused.
- Recent activity in the last 30 days is heavily docs/UX-oriented (e.g., PRs #1223, #1161), suggesting this repository is actively improving user guidance and wording.

---
[What is this?](https://ela.st/github-ai-tools) | [From workflow: Trigger Product Manager Impersonator](https://github.com/elastic/ai-github-actions/actions/runs/30807930275)

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

Start by reading gh-agent-workflows/dependency-review/README.md:46-52, then inspect the two mention workflow README.md and example.yml files named in the issue. Confirm the existing /ai trigger guidance before updating the documentation and comments. Done means both workflows have a Try it now section with 4–6 starter prompts and both examples show commented prompt guidance without changing workflow behavior.

Written by the indexing model from the issue text.