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.

.hadolint.yaml

@@ -0,0 +1,6 @@
+# Container lint policy. The base images are rebuilt on every DSH release, so
+# unpinned apt/pip packages are intentional inside a pinned base image.
+failure-threshold: warning
+ignored:
+  - DL3008  # apt-get packages are intentionally not version-pinned
+  - DL3018  # apk packages are intentionally not version-pinned