migration: add GitHub intake workflows for GitLab handoff

[vivekr-splunk]

This installs the GitHub-side intake surface for the GitHub-to-GitLab migration.

What is included:

• issue intake workflow
• PR intake workflow using metadata-only pull_request_target handling
• shared intake helper scripts under hack/github-intake

Repo-side prerequisites already configured:

• GITLAB_BASE_URL
• GITLAB_PROJECT_PATH
• INTAKE_BACKLINK_MODE=none
• GITLAB_INTAKE_TOKEN

Notes:

• the workflow checks out trusted base-repository automation only
• the PR intake record in GitLab is metadata-only, not a GitLab MR
• backlink comments are disabled by default during migration rehearsal

[rlieberman-splunk]

Is this a one-time run? Or will it be continuous in this repo? If it is staying, maybe we should choose a different folder name instead of hack. Maybe we can move this to tools or create a new gitlab-migration folder.

[vivekr-splunk]

Is this a one-time run? Or will it be continuous in this repo? If it is staying, maybe we should choose a different folder name instead of hack. Maybe we can move this to tools or create a new gitlab-migration folder.

this is intended to stay as the steady-state GitHub intake path after cutover, not a one-time migration run. I moved it out of hack/ into tools/github-intake/ so it reads as ongoing repo automation rather than temporary migration scaffolding.