build: point the release surfaces at RELEASE.2026-08-06

Bundle mcli RELEASE.2026-08-06T00-00-00Z in the container image (pin
and both architecture digests; interop with the new server was
verified end to end during release acceptance) and align the CI
fixture default in install-mcli.sh.

Point the Helm chart defaults at the first real silo image tag:
docker.io/pgsty/silo is a fresh repository, so the inherited
RELEASE.2026-08-04 default could never pull. Chart version moves to
7.0.1 with appVersion RELEASE.2026-08-06T00-00-00Z; the packaging
tripwire in verify-helm-migration.sh follows.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

helm/silo/Chart.yaml

@@ -1,8 +1,8 @@
 apiVersion: v1
 description: S3-Interface Libre Object Storage
 name: silo
-version: 7.0.0
-appVersion: RELEASE.2026-08-04T00-00-00Z
+version: 7.0.1
+appVersion: RELEASE.2026-08-06T00-00-00Z
 keywords:
   - silo
   - storage