build: the cloud frontend stands alone

The Durable Object, the sandbox daemon, and their deploy assets
leave the rhema workspace for a repository of their own. The DO
was already a deployment artifact (publish = false, worker feature
wasm-only), and rhema-sandbox-server's only consumer was the image
beside it — both are product, not framework. The harness and the
seam stay behind: rhema-cloud-agent and the sandbox family join
the rhema repository's publish list, and this tree consumes them
from crates.io at 0.2.6, the same arrangement the claw keeps its
framework crates under.

The sandboxd image build carries the fix its extraction needed:
the old Dockerfile still staged the claw's workspace (claw and
tools manifests, integrations globs, the vendored crossterm) and
missed rhema-provider-registry's manifest entirely. The manifest
stage is now this workspace's two members, fetched from the
registry.

Not yet buildable: the rhema 0.2.6 batch (rhema-sandbox,
-local, -http, rhema-cloud-agent) must publish first — then one
cargo check generates Cargo.lock and CI goes green.

.gitignore

@@ -0,0 +1,5 @@
+/target
+Cargo.lock.bak
+*.snap
+.snapcraft/
+.zcode/