feat(build): raise BuildKit cache-mount budget for remote builders (#112)

BuildKit's default GC caps the ephemeral cache tier — `RUN --mount=type=cache` mounts, local build context, git checkouts — at a hardcoded **512 MB** (shown as `488.3 MiB` in `buildx inspect`).
Across our 7-distro build matrix that fills instantly and forces re-downloads of system + R packages on every rebuild.

## Changes

- **`docker/buildkitd.toml`** (new) — GC config passed via `--config` when creating the `docker-container` builders `artemis` (amd64) and `gaia` (arm64):
  - cache-mount tier: **512 MB → 8 GB**, retained 7 days
  - total cache bounded at **40 GB** with **20 GB min-free**
- **`justfile`** — document the `--config docker/buildkitd.toml` flag on the builder-create commands so a recreate does not silently revert to the 512 MB default.

## Notes

- Limits are absolute (not `%`) because the two hosts differ ~6× in free space (Hetzner ~42 GiB free vs Mac mini ~279 GiB). The 20 GB min-free is the safety valve on the disk-tight Hetzner host; the old default wanted 64 GiB free, which does not exist there.
- Both live builders were already recreated with this config and verified running.

Reviewed-on: #112

justfile

@@ -10,8 +10,10 @@
 #   - buildx builders named `artemis` (amd64) and `gaia` (arm64), created with the
 #     docker-container driver (runs BuildKit on the remote host's docker daemon over
 #     SSH). The default `remote` driver does NOT work with an ssh:// docker host.
-#       docker buildx create --name artemis --driver docker-container ssh://<user@host-amd64>
-#       docker buildx create --name gaia    --driver docker-container ssh://<user@host-arm64>
+#     Pass --config docker/buildkitd.toml so BuildKit's GC keeps a usable cache
+#     (the default caps the cache-mount tier at 512 MB, forcing re-downloads).
+#       docker buildx create --name artemis --driver docker-container --config docker/buildkitd.toml ssh://<user@host-amd64>
+#       docker buildx create --name gaia    --driver docker-container --config docker/buildkitd.toml ssh://<user@host-arm64>
 #   - exported secrets: B2_S3_ACCESS_KEY, B2_S3_SECRET_KEY, PGPASS (GITHUB_PAT optional)
 #
 # Overridable (env or `just VAR=… rebuild …`):