[FEATURE] Make the manifest source tree a valid kustomize base without render #3681

The source manifests only work through ate-setup: images are ko:// references and configuration is spliced in as text. Replace the placeholders and splices with kustomize image overrides and Components, so kubectl kustomize manifests/ate-install/... produces a usable install.

Tracks "Make the source tree usable without the render step" in the Publish the install as kustomize bases workstream of the Extensible Install Roadmap.

Required changes

• Replace ko:// references with plain image names, overridden by images: in kustomize and by render.
• Replace text placeholders and Go-side splices (egress markers, settings files, preview arguments, Postgres storage class) with Components or settings objects.
• Rendered output must not change.

Dependencies: follows #3240 and #3678. Last phase before 1.0.

Enforcement: CI asserts that ate-setup render output is byte-identical before and after each change, and that kubectl kustomize of the source tree with image overrides matches the render.

Parent Issue: #3169