Merge pull request 'preview.env.example: flotilla-core overlay set (retire shim ref)' (#29) from fix/preview-env-flotilla-core into main

deploy/preview/preview.env.example

@@ -5,7 +5,7 @@
 #
 #     while IFS='=' read -r k v; do
 #       [ -n "$k" ] && case "$k" in \#*) ;; *) \
-#         ohm-rfc-app-flotilla overlay set ohm-rfc-app "$k=$v" --preview ;; esac
+#         flotilla-core overlay set <deployment> "$k=$v" --preview ;; esac
 #     done < deploy/preview/preview.env.example
 #
 # CRITICAL (§15 / §3 invariant 1): a preview resolves ZERO real secret bytes.