refactor(ci): route workflows by agent group label instead of named agent

Replace the per-agent `agent: ${AGENT}` label with a group label
`group: rpkgs-${ARCH}`, so workflows target the rpkgs-amd64 / rpkgs-arm64
agent pools rather than a specific agent. Drops the now-unused AGENT matrix
variable across all build/process/weekly workflows.

.crow/build-all-versions.yaml

@@ -1,7 +1,7 @@
 ### Manual full rebuild of all CRAN binaries for one OS/arch.
 # Set the variables below in the manual-run form (or via --var on the CLI).
 # `target_arch` routes the matrix; OS/OS_VERSION/R_VERSION select the platform
-# image and cache volume. Placement is via the agent label (artemis/gaia).
+# image and cache volume. Placement is via the group label (rpkgs-amd64/rpkgs-arm64).
 # Skip list lives in local/excluded-packages.json (read by local/build-all.R).
 variables:
   target_arch:
@@ -31,47 +31,39 @@ skip_clone: true
 
 labels:
   platform: linux/${ARCH}
-  agent: ${AGENT}
+  group: rpkgs-${ARCH}
 
 matrix:
   include:
     - ARCH: amd64
-      AGENT: artemis
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 1
     - ARCH: amd64
-      AGENT: artemis
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 2
     - ARCH: amd64
-      AGENT: artemis
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 3
     - ARCH: amd64
-      AGENT: artemis
       R_PKG_CACHE_DIR: ''
       SPLIT_INTO: 4
       SPLIT_INDEX: 4
     - ARCH: arm64
-      AGENT: gaia
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 1
     - ARCH: arm64
-      AGENT: gaia
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 2
     - ARCH: arm64
-      AGENT: gaia
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 3
     - ARCH: arm64
-      AGENT: gaia
       R_PKG_CACHE_DIR: /mnt/cache/pkgcache
       SPLIT_INTO: 4
       SPLIT_INDEX: 4