MIGRATION.md: the process moved to the host repo

How to bring an existing repo under this methodology, and how to upgrade one that adopted an earlier revision, now lives as a single instruction file in its own repo:

https://github.com/connollydavid/host

Point an agent or a patient human at that URL and follow it. The host file owns the adopt / migrate / upgrade procedure.

This template owns the techniques that procedure installs (the CLAUDE.md spine, the room scaffold, the tool pins) and the revision-keyed upgrade ledger, UPGRADING.md. The procedure is stable and lives in one place; the ledger changes with the techniques that generate it, so it stays here with them.