Fix: lowercase repository owner for GHCR image names

GHCR requires all-lowercase image names; CypherpunkArmory has mixed
case so the tag was being rejected. Lowercase the owner in shell.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: corbinlc

.github/workflows/build.yml

@@ -25,8 +25,9 @@ jobs:
       - name: Compare upstream digest with last-built label
         id: check
         env:
-          GHCR_IMAGE: ghcr.io/${{ github.repository_owner }}/userland-debian
+          OWNER: ${{ github.repository_owner }}
         run: |
+          GHCR_IMAGE="ghcr.io/$(echo "$OWNER" | tr '[:upper:]' '[:lower:]')/userland-debian"
           UPSTREAM=$(docker manifest inspect debian:latest 2>/dev/null \
             | sha256sum | cut -d' ' -f1)
           echo "upstream_digest=$UPSTREAM" >> "$GITHUB_OUTPUT"
@@ -96,7 +97,7 @@ jobs:
           DATE: ${{ steps.date.outputs.value }}
           OWNER: ${{ github.repository_owner }}
         run: |
-          GHCR="ghcr.io/${OWNER}/userland-debian"
+          GHCR="ghcr.io/$(echo "$OWNER" | tr '[:upper:]' '[:lower:]')/userland-debian"
           {
             echo "${GHCR}:latest"
             echo "${GHCR}:${DATE}"