chore(receiver): packaging, k8s manifests, CI/release, docs

Dockerfile.receiver (CGO-free, /data volume), receiver image in CI and
tag releases, receiver binary in release archives, make build-receiver,
example k8s manifests (deployment/service/ingress/secret/pvc) under
deploy/k8s/receiver/, and README coverage including sender/receiver
token pairing.

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

deploy/k8s/receiver/namespace.yaml

@@ -0,0 +1,6 @@
+# TEMPLATE — optional. Skip this file (and drop the namespace fields from the
+# other manifests) to deploy into an existing namespace.
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: exploredns-receiver