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/weekly-rebuild-missing.yaml

@@ -6,7 +6,7 @@
 #   - manual: `weekly_rebuild_missing` dropdown, default "all" (matches the
 #             previous bare manual trigger that ran every os/arch); pick a
 #             single <os>-<arch> to run just one.
-# 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:
   weekly_rebuild_missing:
     description: "Manual run target: a specific <os>-<arch>, or 'all' for every os/arch."
@@ -37,78 +37,64 @@ 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: alpine-322
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.5.3
       IMG: alpine:3.24
     - OS: alpine-323
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.5.3
       IMG: alpine:3.24
     - OS: alpine-323
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.5.3
       IMG: alpine:3.24
     - OS: redhat-8
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: redhat:8
     - OS: redhat-8
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: redhat:8
     - OS: redhat-9
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: redhat:9
     - OS: redhat-9
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: redhat:9
     - OS: redhat-10
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.5.3
       IMG: redhat:10
     - OS: redhat-10
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.5.3
       IMG: redhat:10
     - OS: ubuntu-2204
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: ubuntu:jammy
     - OS: ubuntu-2204
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: ubuntu:jammy
     - OS: ubuntu-2404
       ARCH: amd64
-      AGENT: artemis
       R_VERSION: 4.4.3
       IMG: ubuntu:noble
     - OS: ubuntu-2404
       ARCH: arm64
-      AGENT: gaia
       R_VERSION: 4.4.3
       IMG: ubuntu:noble