[product-manager-impersonator] Multi-label support for scheduled audit findings

## Feature Idea

**Summary:** Add an optional `issue-labels` input (comma-separated) to scheduled audit workflows so created issues can carry multiple repo-specific labels for routing and automation.

## Why a Customer Would Want This
Teams using these detectors often triage by multiple labels (`team/*`, `area/*`, `priority/*`). Today scheduled audits accept only one `issue-label`, which makes downstream routing and dashboarding less precise unless users add manual follow-up automation.

## Rough Implementation Sketch
- Add `issue-labels` (comma-separated) to `.github/workflows/gh-aw-scheduled-audit.md` and propagate it through detector workflows that inherit scheduled-audit behavior.
- Parse and validate each requested label with `github-get_label`, then pass valid labels to `create_issue`.
- Keep backward compatibility: continue honoring `issue-label` (or map it into the list when both are present).
- Update `gh-agent-workflows/scheduled-audit/README.md` and `example.yml` with a multi-label example.

## Why It Won't Be That Hard
This is mostly input-and-plumbing work in an existing path: label handling already exists, safe output already supports multiple labels, and similar comma-separated label parsing already exists in issue triage configuration.

## Evidence
- `.github/workflows/gh-aw-scheduled-audit.md` currently exposes a single `issue-label` input (`lines 36-40`) and single-label guidance (`line 124`).
- `gh-agent-workflows/scheduled-audit/README.md` documents only one `issue-label` input (`line 39`).
- `gh-agent-workflows/scheduled-audit/example.yml` shows only single-label usage (`line 19`).
- `.github/workflows/gh-aw-issue-triage.md` already uses a comma-separated label input pattern (`classification-labels`, `lines 46-50`), providing a reusable implementation approach.
- `claude-workflows/generate-report/ro/action.yml` already supports `issue-labels`, showing precedent for multi-label issue creation in this repository's workflow ecosystem.

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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/30998944668)

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

Start with .github/workflows/gh-aw-scheduled-audit.md, then compare the comma-separated classification-labels handling in .github/workflows/gh-aw-issue-triage.md and issue-labels support in claude-workflows/generate-report/ro/action.yml. Update the scheduled-audit workflow and inherited detector workflows, document the option in gh-agent-workflows/scheduled-audit/README.md and example.yml, and verify single-label compatibility plus validated multi-label issue creation.

Written by the indexing model from the issue text.