[product-manager-impersonator] Configurable activation reaction

## Feature Idea

**Summary:** Let repositories choose the reaction added when an agent workflow activates, while preserving `eyes` as the default.

## Why a Customer Would Want This
Teams use reactions as lightweight status signals, and different repositories already standardize on `rocket`, `heart`, or `+1` rather than the eyes reaction. Today every supported mention/triage workflow imposes `eyes`, so adopting the workflow can conflict with a team's notification and bot-convention preferences.

## Rough Implementation Sketch
- Add an optional `activation-reaction` workflow input with default `eyes` to the mention-in-issue, mention-in-pr, and issue-triage sources, and wire it to the existing `reaction` frontmatter field.
- Validate the input against the reaction values already documented for `add_reaction.cjs` (`+1`, `-1`, `laugh`, `confused`, `heart`, `hooray`, `rocket`, `eyes`).
- Mirror the input in the workflow READMEs/examples and legacy Claude variants where activation reactions are exposed.
- Run `make compile` so generated locks and triggers stay synchronized.

## Why It Won't Be That Hard
The runtime already supports the full reaction set and reads the default from `GH_AW_REACTION`; the workflow sources currently hard-code only one value. This is a small, backward-compatible input/documentation change across existing workflow templates, with no new service or dependency.

## Evidence
- `.github/workflows/gh-aw-mention-in-issue.md:65`, `.github/workflows/gh-aw-mention-in-pr.md:82`, and `.github/workflows/gh-aw-issue-triage.md:64` hard-code `reaction: "eyes"`.
- `scripts/gh-aw-safe-outputs-validation-rules.md:69-76` documents activation reactions and the supported values, including the `eyes` default.
- `gh-agent-workflows/mention-in-issue/README.md:25-32` and `gh-agent-workflows/mention-in-pr/README.md:26-33` show the public input tables but expose no reaction setting.
- Issue searches found no open or prior `[product-manager-impersonator]` request for configurable activation reactions; closed #1267 concerns a permissions regression, not this feature.

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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/29573763997)

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

Start with the hard-coded reaction entries in .github/workflows/gh-aw-mention-in-issue.md, gh-aw-mention-in-pr.md, and gh-aw-issue-triage.md, then compare the public inputs in the corresponding gh-agent-workflows README files. Use the documented reaction values and update the legacy Claude variants mentioned in the issue; run make compile, with done meaning the inputs are exposed, eyes remains the default, and generated locks and triggers are synchronized.

Written by the indexing model from the issue text.