[product-manager-impersonator] Add workflow-upgrade-assistant to auto-migrate deprecated gh-aw workflow refs

Add a new small reusable workflow: `gh-aw-workflow-upgrade-assistant.lock.yml`.

It should scan consumer `.github/workflows/*.yml` for deprecated `elastic/ai-github-actions` workflow refs and open a PR with safe, mechanical upgrades (e.g., `gh-aw-stale-issues` → `gh-aw-stale-issues-investigator`, `mention-in-issue-by-id` → `create-comment-on-issue`, removed dedicated fixers → detector+`create-pr-from-issue` chain snippet).

### Why this is customer-value
Users regularly lag on migrations and keep deprecated workflow names for compatibility. A one-click upgrader removes manual YAML hunting and reduces breakage risk.

### Evidence in repo
- `docs/upgrading.md` documents many rename/deprecation mappings and examples.
- `docs/workflows/detector-fixer-chaining.md` already defines canonical replacement chaining mechanics.
- `scripts/rewrite_setup_action_refs.py` shows precedent for automated ref rewriting of workflow files.
- Recent direction already introduced `create-comment-on-issue` while preserving deprecated paths.

### Why it won’t be that hard
Mostly deterministic text/AST rewrites plus a migration map already documented. Existing issue→PR automation patterns can be reused (`create-pr-from-issue` style flow and safe outputs).

### Small-scope acceptance criteria
1. New reusable workflow runs on `workflow_dispatch`.
2. Detects deprecated ai-github-actions workflow refs in `.github/workflows`.
3. Creates one PR with upgraded refs + minimal inline migration notes for non-1:1 cases.
4. Emits `noop` when no changes are needed.

### Duplicate-risk check (quick)
Low. Existing ideas focused on preflight/lint/watchdog/noop ergonomics; this is a user-facing migration assistant capability for workflow consumers, not a lint/check utility.

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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/23338245621)

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