- Accept TF_VAR_-prefixed B2 keys (from direnv); make GITHUB_PAT optional.
- Stream the remote build with --progress=plain.
- Document creating the artemis/gaia buildx builders with the docker-container
driver (the default remote driver does not work with an ssh:// docker host).
## Summary
Follow-up fix to the `just rebuild` recipe (#87): the local `.envrc` exports the B2 credentials as `TF_VAR_B2_S3_ACCESS_KEY` / `TF_VAR_B2_S3_SECRET_KEY` (and has no `GITHUB_PAT`), but the recipe required the plain `B2_S3_ACCESS_KEY` / `B2_S3_SECRET_KEY` names and always passed a `github_pat` secret.
- Accept the `TF_VAR_`-prefixed B2 names (falling back to the plain names if set directly).
- Pass the `github_pat` buildx secret only when `GITHUB_PAT` is set, so the build works without it.
Reviewed-on: #88
## Summary
Adds a local `just rebuild` recipe to (re)build specific versions of a single package on a given OS/arch, dispatching to a remote buildx builder (the build runs there, not locally).
- `just rebuild <os> <tag> <arch> <package> <version>...` → `docker buildx build --builder <artemis|gaia> --platform linux/<arch> …` (amd64→artemis, arm64→gaia; names + `R_VERSION` env-overridable).
- `docker/build-one.Dockerfile` runs `build-one.R` as a secret-mounted `RUN`, built `--no-cache --output type=cacheonly` (pure side-effect: the S3 upload; no image kept).
- `local/build-one.R` auto-classifies each version via the ABI classifier (risky → per-minor slot `contrib/<x.y>/`, else generic), force-rebuilds + uploads + stores metadata, then refreshes the touched slot's `PACKAGES` index.
## Prerequisites
- buildx builders named `artemis` (amd64) and `gaia` (arm64) registered (`docker buildx create --name artemis ssh://…`).
- Exported secrets: `B2_S3_ACCESS_KEY`, `B2_S3_SECRET_KEY`, `PGPASS` (`GITHUB_PAT` optional).
- bincraft `v4.2.0` tag must exist (the build installs `@v4.2.0` and uses its classifier + per-minor index API).
Reviewed-on: #87
