Create GitHub Actions workflow templates for MGW pipeline

[snipcodeit]

Description

Add .github/workflows/mgw-auto-triage.yml (on issue opened) and mgw-milestone-exec.yml (workflow_dispatch) as opt-in templates. Document how users copy to their own repos.

Acceptance Criteria

• Create GitHub Actions workflow templates for MGW pipeline

GSD Route

plan-phase

Phase Context

Phase 30: Headless Mode and GitHub Actions Integration of v4 — Standalone and CI Usage

Depends on

#implement---headless-flag-in-mgw-run-and-mgw-issue