fix: unblock instanceable connectivity store-service install + clean up on delete

Live verification on pic1 of the connectivity v2 multi-instance feature
surfaced four integration bugs that prevented installing any published
connectivity store service (proxy/wireguard-ext/openvpn-client/sshuttle)
and left stale host routing state behind. All four are fixed here:

1. manifest_validator rejected the CI-published `name:tag@sha256:<digest>`
   image form (it required digest-only), while service_store_manager already
   accepted it — so every published store image failed validation. Allow an
   optional tag before the digest, matching service_store_manager.

2. The cell-api image shipped the docker CLI but not the Compose v2 plugin,
   so every `docker compose` ServiceComposer runs (pull/up/down for store
   services) failed with "'compose' is not a docker command". Copy the
   compose plugin binary from the docker-cli stage.

3. service_store_manager.install ran the base compose up for instanceable
   services, whose template still contains ${INSTANCE_ID}/${REDIRECT_PORT}
   (there is no base container — one runs per connection instance). It now
   verifies the image signature but defers the container to connection
   creation for instanceable manifests.

4. delete_connection freed the record/secrets/container but never removed the
   connection's individually-managed `ip rule fwmark->table` or its FORWARD
   kill-switch (apply_routes only flushes the PIC_CONNECTIVITY chains and
   re-adds rules for surviving connections), leaking stale host routing state.
   It now tears both down; added _remove_killswitch.

Verified end-to-end on pic1: two proxy instances allocate distinct
marks/tables/ports (skipping in-use resources), render distinct per-instance
containers, two peers route through distinct instances (per-peer MARK +
REDIRECT), delete is blocked while referenced (409) and cleans its ip rule.

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

api/Dockerfile

@@ -8,7 +8,11 @@ WORKDIR /app/api
 
 # The API runs as root by design: it drives iptables, the docker socket, and
 # docker-execs into sibling containers. Non-root is not feasible here.
+# The Compose v2 plugin is a separate binary under cli-plugins/ — ServiceComposer
+# shells out to `docker compose` for every store-service lifecycle op, so it must
+# be copied alongside the docker CLI, not just the docker binary.
 COPY --from=dockercli /usr/local/bin/docker /usr/local/bin/docker
+COPY --from=dockercli /usr/local/libexec/docker/cli-plugins/docker-compose /usr/local/libexec/docker/cli-plugins/docker-compose
 
 # cosign verifies store-service image signatures against the bundled public key
 # (config/cosign/cosign.pub) before ServiceComposer starts a container.