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/71-httproute.yaml

@@ -0,0 +1,23 @@
+apiVersion: gateway.networking.k8s.io/v1
+kind: HTTPRoute
+metadata:
+  name: dsh-gateway
+  namespace: dsh-demo
+  labels:
+    app.kubernetes.io/part-of: dsh-demo
+spec:
+  parentRefs:
+    - name: dsh-gateway
+      namespace: dsh-demo
+      sectionName: https
+  hostnames:
+    # TODO: replace with the actual public FQDN before applying.
+    - dsh-demo.example.com
+  rules:
+    - backendRefs:
+        # Route through oauth2-proxy (GitHub OAuth) first; it authenticates the
+        # browser and forwards to dsh-gateway:8080 with X-Forwarded-User set.
+        # The gateway only trusts that header when AUTH_PROXY_HEADER is set on
+        # its Deployment — see 72-oauth2-proxy.yaml and ../README.md.
+        - name: oauth2-proxy
+          port: 4180