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

[github-actions]

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? | From workflow: Trigger Product Manager Impersonator

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