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-install-deps.yaml

@@ -30,14 +30,12 @@ skip_clone: true
 
 labels:
   platform: linux/${ARCH}
-  agent: ${AGENT}
+  group: rpkgs-${ARCH}
 
 matrix:
   include:
     - ARCH: amd64
-      AGENT: artemis
     - ARCH: arm64
-      AGENT: gaia
 
 steps:
   - name: 'Install deps and bincraft'