refactor(ci): route workflows by agent group label instead of named agent (#94)

## Summary

- Switch crow workflow placement from a per-agent label (`agent: ${AGENT}`) to a group label (`group: rpkgs-${ARCH}`), so jobs target the `rpkgs-amd64` / `rpkgs-arm64` agent pools instead of a single named agent (`artemis`/`gaia`).
- Remove the now-unused `AGENT` matrix variable from all build/process/weekly workflows.
- Update header comments to reflect group-based placement.

Agents opt into a pool via `CROW_AGENT_LABELS="group=rpkgs-<arch>"`. The group name derives from each matrix row's `ARCH`, so adding or replacing agents no longer requires touching the workflow files.

Reviewed-on: #94

.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