[product-manager-impersonator] Threaded replies default for mention workflows

## Feature Idea

**Summary:** Add a small threading control input to `mention-in-pr` and `mention-in-issue` so replies can consistently target the triggering comment via `add_comment.reply_to_id` instead of always creating top-level comments.

## Why a Customer Would Want This
Teams using `/ai` mention workflows want less timeline noise and clearer conversation context. Threaded replies keep the bot response attached to the exact request, which is easier to follow in busy PRs/issues and avoids comment pileups.

## Rough Implementation Sketch
- Add a new optional workflow input (for example `reply-in-thread`) in the source workflows for:
- `.github/workflows/gh-aw-mention-in-pr.md`
- `.github/workflows/gh-aw-mention-in-issue.md`
- Update the prompt instructions so when enabled, the agent sets `reply_to_id` to the triggering comment ID on `add_comment`.
- Recompile lock workflows and update workflow READMEs to document the new behavior and default.
- Keep behavior backward-compatible (opt-in or carefully chosen default) so adopters can roll this out safely.

## Why It Won't Be That Hard
This mostly wires existing capabilities together: safe outputs for both mention workflows already accept `reply_to_id`, so the work is confined to prompt/input plumbing and docs.

## Evidence
- Mention workflow inputs currently have no threading control:
- `gh-agent-workflows/mention-in-pr/README.md#L26-L33`
- `gh-agent-workflows/mention-in-issue/README.md#L25-L32`
- Prompt instructions currently just tell the agent to call `add_comment`:
- `.github/workflows/gh-aw-mention-in-pr.md#L208-L211`
- `.github/workflows/gh-aw-mention-in-issue.md#L133-L136`
- Safe outputs already support threaded replies (`reply_to_id`) in these workflows:
- `.github/workflows/gh-aw-mention-in-pr.lock.yml#L844-L845` and `#L867-L870`
- `.github/workflows/gh-aw-mention-in-issue.lock.yml#L706-L707` and `#L727-L730`
- Recent product signal that comment-noise reduction matters:
- Issue: https://github.com/elastic/ai-github-actions/issues/1098
- PR: https://github.com/elastic/ai-github-actions/pull/1099

> [!NOTE]
>
> 🔒 Integrity filter blocked 47 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - [#937](https://github.com/elastic/ai-github-actions/pull/937) `search_pull_requests`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#1067](https://github.com/elastic/ai-github-actions/issues/1067) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#359](https://github.com/elastic/ai-github-actions/issues/359) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#704](https://github.com/elastic/ai-github-actions/issues/704) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#694](https://github.com/elastic/ai-github-actions/issues/694) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#684](https://github.com/elastic/ai-github-actions/issues/684) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#875](https://github.com/elastic/ai-github-actions/issues/875) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#676](https://github.com/elastic/ai-github-actions/issues/676) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#321](https://github.com/elastic/ai-github-actions/issues/321) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#132](https://github.com/elastic/ai-github-actions/issues/132) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#139](https://github.com/elastic/ai-github-actions/issues/139) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#225](https://github.com/elastic/ai-github-actions/issues/225) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#336](https://github.com/elastic/ai-github-actions/issues/336) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#1025](https://github.com/elastic/ai-github-actions/issues/1025) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#354](https://github.com/elastic/ai-github-actions/issues/354) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - elastic/ai-github-actions#1067 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - ... and 31 more items
>
> To allow these resources, lower `min-integrity` in your GitHub frontmatter:
>
> ```yaml
> tools:
> github:
> min-integrity: approved # merged | approved | unapproved | none
> ```
>
>

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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/25794426415)

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