Channel-correct Docker images: keyring from own channel; repo-add uses edge

The builder stage never declared ARG MIRROR, so the keyring [omarchy] repo
pointed at the channel-less legacy pkgs.omarchy.org/$arch path — it works
only because a stale copy of the old layout still answers there, and it would
miss a keyring rotation. Each image now pulls omarchy-keyring from its own
channel (edge/rc/stable), matching the base mirror it already selects.

update-repo and remove-package switch to the edge x86_64 image: repo-add and
repo-remove compile nothing, and using the channel image would deadlock
bootstrap-rc — the rc image can only build once the rc channel it pulls the
keyring from exists remotely.

bin/remove-package

@@ -89,7 +89,8 @@ if [[ ! $REPLY =~ ^[Yy]$ ]]; then
 fi
 
 # Build/update the Docker image (always use x86_64 for removal - it's architecture independent)
-build_docker_image "$BUILD_DIR" "x86_64" "$MIRROR"
+# repo-remove is mirror-independent — always use the edge x86_64 image
+build_docker_image "$BUILD_DIR" "x86_64" "edge"
 
 acquire_release_lock || exit 1
 
@@ -104,7 +105,7 @@ docker run --rm --platform linux/amd64 \
   -e MIRROR="$MIRROR" \
   -v "$REPO_ROOT:/pkgs.omarchy.org" \
   -v "$BUILD_DIR:/build:ro" \
-  omarchy-pkg-builder:latest-x86_64-$MIRROR /build/remove-package.sh "$PACKAGE_NAME"
+  omarchy-pkg-builder:latest-x86_64-edge /build/remove-package.sh "$PACKAGE_NAME"
 
 RESULT=$?