Proposal: add migrate.yaml skeleton

[Interested-Deving-1896]

Upstream workflow proposal

Source: Interested-Deving-1896/api-base/.github/workflows/migrate.yaml

This workflow was detected in an OSP-bound repo and does not yet exist in fork-sync-all.
The file has been sanitised:

• Hardcoded org/repo names replaced with generic placeholders
• Cron schedules marked with # TODO comments
• Project-specific run-name lines removed

Review checklist before merging:

• Workflow is genuinely reusable across projects (not repo-specific logic)
• No hardcoded repo or org names remain in run: shell blocks (sanitisation covers env:, with:, and default: fields but not inline shell strings)
• Cron schedule is appropriate for a template (or removed entirely)
• workflow_run: trigger names updated or removed (replaced with TODO by sanitisation)
• Add to the allowlist in scripts/validate-workflows.sh