Document the transferred-repo pattern in `platform/README.md`

[taprile314]

Part of #20

Scope

Document the transferred flag in platform/README.md: when to use it, how to set it in a caller workflow, and how to verify the deploy was skipped.

Acceptance Criteria

• README section explains when to apply the flag (after a repo handoff to a client Vercel team).
• README shows a minimal caller snippet passing transferred: true.
• README documents the skip log line so operators can verify the skip happened.