Add rc as a first-class channel: validation, shared repo root, rc build mirror

- helpers/paths.sh: validate_mirror/require_valid_mirror for the edge|rc|stable
  set, and REPO_ROOT (OMARCHY_REPO_ROOT override) so a secondary checkout like
  the rc branch worktree publishes into the same channel tree as the primary
- validate --mirror everywhere it previously accepted any string (sync-repo,
  promote-build, update-repo, clean-repo, remove-package) and widen the
  edge|stable checks in build, deploy, push-build, auto-release
- build/Dockerfile: rc builds compile against rc-mirror.omarchy.org

bin/deploy

@@ -31,8 +31,8 @@ while [[ $# -gt 0 ]]; do
     ;;
   --mirror)
     MIRROR="$2"
-    if [[ "$MIRROR" != "edge" && "$MIRROR" != "stable" ]]; then
-      print_error "Invalid mirror: $MIRROR (must be 'edge' or 'stable')"
+    if ! validate_mirror "$MIRROR"; then
+      print_error "Invalid mirror: $MIRROR (must be one of: $VALID_MIRRORS)"
       exit 1
     fi
     update_arch_paths
@@ -69,7 +69,7 @@ while [[ $# -gt 0 ]]; do
     echo ""
     echo "Options:"
     echo "  --arch <arch>          Target architecture (default: x86_64)"
-    echo "  --mirror <mirror>      Mirror to publish to (edge or stable, default: edge)"
+    echo "  --mirror <mirror>      Mirror to publish to (edge, rc, or stable, default: edge)"
     echo "  --package <names>      Build and push only these packages (space-separated)"
     echo "  --host <host>          ssh destination (default: \$OMARCHY_REPO_HOST or .repo-host)"
     echo "  --remote-root <path>   Repository path on the host (default: /root/omarchy-pkgs)"