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/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