Slim admin-editor example and docs for EmailTemplateAdmin embed

[patoperpetua]

Parent: #138
Depends on: #140

Goal

Make EmailTemplateAdmin the default embedding model: thin examples/admin-editor to mount that one component plus memory/BFF adapters, and rewrite docs/guides/editor-integration.md accordingly.

Scope

• Update examples/admin-editor/App.tsx (and README) to mount EmailTemplateAdmin instead of hand-rolled list + EmailTemplateEditor.
• Rewrite docs/guides/editor-integration.md primary path around one-component embed; keep a short advanced note for EmailTemplateEditor.
• Update cross-links in README / overview / onboarding if they still describe only the low-level editor.
• Comment on epic Epic: Full embeddable EmailTemplateAdmin page component #138 when done and close the epic if all children are satisfied.

Constraints

• Do not add a hosted CMS under apps/.
• Do not put secrets in the example browser bundle.

Acceptance criteria

• Example mounts EmailTemplateAdmin with catalog + onSave (+ optional send-test BFF pattern).
• Editor integration guide leads with the one-component model.
• Example tests pass.
• Epic checklist items are completed / epic closed with a summary comment.

Agent implementation notes

Branch: feat/<N>-admin-emailtemplateadmin-docs.