Remove the cold-build workflow: Yocto builds on the build host only

The yocto-cold-build workflow and its kas/ci.yml overlay built the release image on a hosted runner as a reproducibility probe. It never ran to completion, its first dispatch (2026-09-09) stopped on the runner's user-namespace rule, and a probe nobody runs is a trap. Every Yocto build, the release included, runs on the build host; the release proof is the local pipeline (release.sh) and the bench campaign. The pre-publish checklist loses its self-containment line to match.

kas/ci.yml

@@ -1,16 +0,0 @@
-# CI overlay for the cold-build reproducibility workflow: merge after the
-# main config (kas build kas/forgefirm-glowforge.yml:kas/ci.yml).
-# rm_work keeps the build inside a hosted runner's disk budget; the
-# release image alone is built (the dev image adds nothing to the
-# reproducibility question).
-
-header:
-  version: 14
-
-target: forgefirm-image
-
-local_conf_header:
-  ci: |
-    INHERIT += "rm_work"
-    BB_NUMBER_THREADS = "4"
-    PARALLEL_MAKE = "-j 4"