feat(skills): add migrating-private-integration skill

[TheRealAgentK]

Add a skill that documents how to migrate an integration from the private integrations repo to the public autohive-integrations repo

Triggered by the Gmail migration (see companion PRs in the public and private integrations repos). The process needs to be repeatable and consistent across engineers — particularly the security/safety/secrets review that gates whether something can be published at all.

Scope

• Add skills/migrating-private-integration/SKILL.md with:
  • Pre-migration security/safety/secrets checklist (with explicit categories and ripgrep/grep commands)
  • Structural cleanup steps (drop vendored SDK, legacy harness, switch to public repo conventions)
  • File copy + adapt steps (README rewrite, requirements pin alignment, tests directory)
  • Validation via the tooling repo's validate_integration.py and check_code.py
  • PR coordination across the three repos (public add, private remove, SDK skill)
  • Required prompt to ask the Autohive engineering Slack channel for a second opinion before merging