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

build/Dockerfile

@@ -39,6 +39,8 @@ RUN mkdir -p /etc/pacman.d && \
 RUN if [ "${TARGETARCH}" = "amd64" ]; then \
         if [ "${MIRROR}" = "stable" ]; then \
             printf 'Server = https://stable-mirror.omarchy.org/$repo/os/$arch\n' > /etc/pacman.d/mirrorlist; \
+        elif [ "${MIRROR}" = "rc" ]; then \
+            printf 'Server = https://rc-mirror.omarchy.org/$repo/os/$arch\n' > /etc/pacman.d/mirrorlist; \
         else \
             printf 'Server = https://mirror.omarchy.org/$repo/os/$arch\n' > /etc/pacman.d/mirrorlist; \
         fi; \