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/auto-release

@@ -13,12 +13,12 @@ STATE_DIR="${OMARCHY_STATE_DIR:-/root/.state}"
 
 if [[ -z "$MIRROR" ]]; then
   print_error "Usage: $0 <mirror>"
-  echo "  mirror: edge or stable"
+  echo "  mirror: edge, rc, or stable"
   exit 1
 fi
 
-if [[ "$MIRROR" != "edge" && "$MIRROR" != "stable" ]]; then
-  print_error "Invalid mirror: $MIRROR (must be 'edge' or 'stable')"
+if [[ "$MIRROR" != "edge" && "$MIRROR" != "rc" && "$MIRROR" != "stable" ]]; then
+  print_error "Invalid mirror: $MIRROR (must be 'edge', 'rc', or 'stable')"
   exit 1
 fi