docs(compliance): add repo-transfer-to-enterprise playbook

Canonical best-practice playbook for transferring a private repo from a
personal/shared GitHub org to a GitHub Enterprise Cloud account. Covers
pre-transfer audit (reference scan, submodule dependents, mirror configs,
secrets inventory, branch protection, webhooks, Pages impact, backup),
transfer execution, post-transfer verification + remediation, rollback
plan, and a dense set of gotchas learned from real transfers (secret
value loss, GPG signing, org-level Actions policies, enterprise pooled
minutes, redirect-window breakage).

Intended use: instantiate as a repo-specific audit doc in the target
repo's `audits/` dir, or as a pre-flight entry in YOUR-ACTIONS.md, for
each transfer. This template is the canonical source; repo-specific
instances should link back to it rather than duplicate its content.

First user (not included here): 007 -> Metadatastician enterprise org
transfer, being drafted into YOUR-ACTIONS.md in the same 2026-04-17
session.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath