## Summary
Consolidate the four `build-all-versions*` files into two, mirroring the matrix approach used for the process-updates / weekly families:
| Before | After |
|---|---|
| `build-all-versions-amd64.yaml` + `build-all-versions-arm64.yaml` | `build-all-versions.yaml` |
| `build-all-versions-install-deps-amd64.yaml` + `-arm64.yaml` | `build-all-versions-install-deps.yaml` |
`ARCH` becomes a matrix axis carrying the per-arch differences: `agent` (artemis/gaia), `BACKEND` (kubernetes/docker), and `R_PKG_CACHE_DIR`. Build-all keeps its `SPLIT_INDEX` parallelism, so its matrix is ARCH x SPLIT = 8 rows; only the rows matching the selected arch run.
**Invocation unchanged.** Routing uses the existing task selector `task == "build-all-${ARCH}"`, so `--var task=build-all-amd64 ...` still runs exactly the amd64 rows. `OS` / `OS_VERSION` / `R_VERSION` / `K8S_INSTANCE_TYPE` are still passed via `--var`. (You can drop `--var ARCH=...` now — arch comes from the matrix/task.)
**No cron.** These pipelines are manual-only, so there are no scheduled runs to affect.
## Validation before merge
Same matrix-interpolation-in-`when`/`labels` mechanism validated for #92, plus it now relies on per-row `agent`/`BACKEND` selecting the right execution backend (kubernetes for amd64, docker for arm64). Before merge, on this branch trigger a small `task=build-all-arm64` run and confirm it lands on the gaia/docker backend (and amd64 on artemis/kubernetes). Originals are in git history for rollback.
Note: `archive-missed-packages.yaml` was already a single consolidated matrix file (cron + manual) and is left untouched.
Reviewed-on: #93
