deploy: runbook for updating a running instance ...

Adds deploy/UPDATE.md: preflight, the branch shapes (master vs the
tracked .env on deploy), what to rebuild before pushing (the frontend
dist is committed and the server never builds it), the verification
commands that prove the new bundle is the one being served, rollback,
and the traps — tracked secrets in mcp_servers.d/, gitignored vendor
kit dist, service-worker cache, Android app needing only a restart.

Linked from deploy/README.md.