Add Regression PR Shepherd agentic workflow

Shepherds open AI-Issue-Regression-PR pull requests to completion:
- Fixes CI failures and addresses review feedback
- Detects when a test proves the bug still exists (removes label, tags maintainers)
- Runs 6x/day, only touches tests/ files

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: T-Gro