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/process-updates.yaml

@@ -5,7 +5,7 @@
 #             its matching matrix row (via the per-row `cron:` name filter).
 #   - manual: pick a target from the `process_cran_updates` dropdown
 #             ("all" = every os/arch).
-# Arch placement is handled by the agent label (artemis=amd64, gaia=arm64).
+# Arch placement is handled by the group label (rpkgs-amd64, rpkgs-arm64).
 variables:
   process_cran_updates:
     description: "Manual run target: a specific <os>-<arch>, or 'all' for every os/arch."
@@ -36,104 +36,90 @@ when:
 skip_clone: true
 
 labels:
-  agent: ${AGENT}
+  group: rpkgs-${ARCH}
 
 matrix:
   include:
     - OS: alpine-322
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.5.3
       IMG: alpine:3.24
       OS_ID: alpine322
       PROCESS_NEW: "FALSE"
     - OS: alpine-322
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.5.3
       IMG: alpine:3.24
       OS_ID: alpine322
       PROCESS_NEW: "FALSE"
     - OS: alpine-323
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.5.3
       IMG: alpine:3.24
       OS_ID: alpine323
       PROCESS_NEW: "FALSE"
     - OS: alpine-323
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.5.3
       IMG: alpine:3.24
       OS_ID: alpine323
       PROCESS_NEW: "FALSE"
     - OS: redhat-8
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: redhat:8
       OS_ID: rhel8
       PROCESS_NEW: "TRUE"
     - OS: redhat-8
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: redhat:8
       OS_ID: rhel8
       PROCESS_NEW: "TRUE"
     - OS: redhat-9
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: redhat:9
       OS_ID: rhel9
       PROCESS_NEW: "TRUE"
     - OS: redhat-9
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: redhat:9
       OS_ID: rhel9
       PROCESS_NEW: "TRUE"
     - OS: redhat-10
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.5.3
       IMG: redhat:10
       OS_ID: rhel10
       PROCESS_NEW: "TRUE"
     - OS: redhat-10
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.5.3
       IMG: redhat:10
       OS_ID: rhel10
       PROCESS_NEW: "TRUE"
     - OS: ubuntu-2204
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: ubuntu:jammy
       OS_ID: jammy
       PROCESS_NEW: "TRUE"
     - OS: ubuntu-2204
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: ubuntu:jammy
       OS_ID: jammy
       PROCESS_NEW: "TRUE"
     - OS: ubuntu-2404
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: ubuntu:noble
       OS_ID: noble
       PROCESS_NEW: "TRUE"
     - OS: ubuntu-2404
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: ubuntu:noble
       OS_ID: noble