[product-manager-impersonator] Configurable mention command prefix

## Feature Idea

**Summary:** Let teams configure the slash-command prefix used by the mention workflows through a documented repository or organization variable, while keeping `/ai` as the default.

## Why a Customer Would Want This
Some teams already reserve `/ai`, prefer a more explicit command such as `/copilot`, or want different conventions across repositories. The mention workflow docs already frame the prefix as configurable, so exposing a tiny, copy-pasteable variable hook would make that promise real without asking users to hand-edit the trigger condition every time.

## Rough Implementation Sketch
- Update the mention trigger examples to use a variable fallback, e.g. `startsWith(github.event.comment.body, vars.GH_AW_MENTION_PREFIX || '/ai')`.
- Apply the same pattern to the sandboxed and no-sandbox issue/PR mention examples so behavior stays consistent.
- Document `GH_AW_MENTION_PREFIX` in the Mention in Issue and Mention in PR READMEs, including the default `/ai` behavior and a short example for setting it as an Actions repository variable.
- Run the existing compile/docs flow so generated trigger copies and docs stay in sync.

## Why It Won't Be That Hard
This is a small trigger-template and documentation change. The reusable workflows already receive the comment body after the caller trigger has decided to run, so the feature does not require changes to safe outputs, agent prompts, permissions, or workflow internals.

## Evidence
- `gh-agent-workflows/mention-in-issue/README.md:7` says the example trigger uses `/ai`, "but the prefix is configurable," yet the example hardcodes `/ai` at `gh-agent-workflows/mention-in-issue/example.yml:16-19`.
- `gh-agent-workflows/mention-in-pr/README.md:7` makes the same configurability claim, while `gh-agent-workflows/mention-in-pr/example.yml:17-20` hardcodes `/ai`.
- The no-sandbox variants also hardcode `/ai`: `gh-agent-workflows/mention-in-issue-no-sandbox/example.yml:16-19` and `gh-agent-workflows/mention-in-pr-no-sandbox/example.yml:18-21`.
- Local search found no existing `GH_AW_MENTION_*`, `MENTION_PREFIX`, or `COMMAND_PREFIX` support in the workflow examples/docs.
- Duplicate check: prior Product Manager Impersonator reports include quick setup defaults, trigger linting, workflow outputs, run manifests, and persona overrides, but no existing report for configurable mention command prefixes.

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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/28165180268)

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Start with the trigger examples at gh-agent-workflows/mention-in-issue/example.yml, mention-in-pr/example.yml, and their no-sandbox counterparts, then compare the corresponding Mention in Issue and Mention in PR READMEs. Run the existing compile/docs flow to identify generated copies. Done means all examples use the configurable repository or organization variable, retain /ai as the default, and document the setting consistently.

Written by the indexing model from the issue text.