[product-manager-impersonator] Multi-label routing for Scheduled Audit/Fix issues

## Feature Idea

**Summary:** Add an optional `issue-labels` input (comma-separated) to Scheduled Audit and Scheduled Fix so teams can route and pick up findings using multiple labels instead of a single label.

## Why a Customer Would Want This
Teams often route automation issues by more than one dimension (for example `agentic-workflows`, team ownership, and severity). Today these workflows only support one label, which forces manual relabeling and slows triage/fix handoff.

## Rough Implementation Sketch
- Add `issue-labels` (CSV string) to `.github/workflows/gh-aw-scheduled-audit.md` and `.github/workflows/gh-aw-scheduled-fix.md`.
- Keep `issue-label` for backward compatibility, but normalize both inputs to a list internally.
- In Scheduled Audit, apply all valid labels during `create_issue` (checking existence similarly to current `github-get_label` behavior).
- In Scheduled Fix, search/queue issues using the normalized label list (documenting OR/AND matching behavior explicitly).

## Why It Won't Be That Hard
This is a narrow input-plumbing change in two reusable workflows that already have label-aware logic. The implementation reuses existing safe-output and GitHub search patterns with minimal surface area.

## Evidence
- Single-label audit input today: `.github/workflows/gh-aw-scheduled-audit.md:36` and docs table `gh-agent-workflows/scheduled-audit/README.md:39`.
- Audit currently handles only one label in issue creation guidance: `.github/workflows/gh-aw-scheduled-audit.md:121`.
- Single-label fix input and filtering today: `.github/workflows/gh-aw-scheduled-fix.md:35`, `.github/workflows/gh-aw-scheduled-fix.md:126`, `.github/workflows/gh-aw-scheduled-fix.md:129`, and docs table `gh-agent-workflows/scheduled-fix/README.md:34`.
- Duplicate check: no existing open PM issue title match for scheduled label expansion (`repo:elastic/ai-github-actions is:issue in:title "[product-manager-impersonator]" ("scheduled" AND "label")` returned zero). Similar-but-different items include #974 (quick-setup label bootstrap), #935 (no-op reporting mode), and #1086 (previous-findings fetch depth).

> [!NOTE]
>
> 🔒 Integrity filter blocked 23 items
>
> The following items were blocked because they don't meet the GitHub integrity level.
>
> - elastic/ai-github-actions#1067 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#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".
> - [#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".
> - [#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".
> - [#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".
> - elastic/ai-github-actions#1025 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - elastic/ai-github-actions#875 `list_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - [#224](https://github.com/elastic/ai-github-actions/issues/224) `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".
> - [#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".
> - [#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".
> - [#248](https://github.com/elastic/ai-github-actions/issues/248) `search_issues`: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
> - ... and 7 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/25666215217)

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