fix: remove non-standard architectures field from feature JSON

The devcontainers/action schema validator rejects `architectures` as an
unrecognized property. Architecture detection is handled at runtime by
install.sh, so the field was purely informational.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: bradleybeddoes

src/dbr/devcontainer-feature.json

@@ -14,9 +14,5 @@
   "entrypoint": "/usr/local/share/dbr/entrypoint.sh",
   "installsAfter": [
     "ghcr.io/devcontainers/features/common-utils"
-  ],
-  "architectures": [
-    "amd64",
-    "arm64"
   ]
 }