feat(repo)!: restructure around the dsh-harness image and one example per implementation

- the root Dockerfile is the primary artifact; versions.env is the single source of truth
- merge gateway-api/dsh-demo into examples/dsh-agents-as-a-service (manifests/, exposure variants)
- move the single-tenant template into examples/minimal-single-tenant
- publish the gateway as a nested package under the repository image
- mirror the DSH ref for release identities (first release dsh-v0.2.1-rc.2-r2)
- CI: lint/python/manifests/gateway-image, reusable multi-arch build, tag-driven release, weekly e2e
- docs: README, docs/{main-image,examples,gitflow,versioning}, example READMEs, AGENTS.md, governance
- SSO is opt-in again; gateway pull policy and cookie defaults match the documented demo

BREAKING CHANGE: example manifest paths, the gateway image name, and the removal
of deploy/ and gateway-api/ affect existing deployments; see the migration table
in README.md.

examples/dsh-agents-as-a-service/manifests/exposure/gateway-api-sso/70-gateway.yaml

@@ -0,0 +1,25 @@
+# Browser entry point (TLS) for the gateway-api-sso variant: terminates TLS
+# for dsh-demo.example.com and routes through 71-httproute.yaml to
+# oauth2-proxy, which authenticates the browser before the gateway.
+# Mutually exclusive with ../../ingress/ (see ../../README.md). The
+# "dsh-demo-tls" Secret comes from 74-tls-secret.yaml.example.
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  name: dsh-gateway
+  namespace: dsh-demo
+  labels:
+    app.kubernetes.io/part-of: dsh-demo
+spec:
+  gatewayClassName: traefik
+  listeners:
+    - name: https
+      port: 8443
+      protocol: HTTPS
+      tls:
+        mode: Terminate
+        certificateRefs:
+          - name: dsh-demo-tls
+      allowedRoutes:
+        namespaces:
+          from: Same