Add EmailBuilder editor integration guide

[patoperpetua]

Parent: #7
Depends on: #5

Goal

Document how a consumer embeds @singleton-sd/post-kit-editor in a React admin application so an internal user can edit EmailBuilder.js template sources, preview them, and persist changes through a consumer-controlled persistence adapter that ultimately feeds the Git-backed publish pipeline. This covers the epic's "Admin template editing" scenario and its editor-integration work item.

Scope

Add docs/guides/editor-integration.md

• What the editor package is and is not: an authoring surface for template.json sources; it does not send email, does not hold PostKit send credentials, and does not write to Azure Blob Storage.
• Installation and peer requirements (React version, bundler expectations) as actually shipped by the package.
• A minimal embedding example: mounting the editor component, loading an existing template.json, and receiving edited output.
• The persistence adapter contract as implemented: the interface the consumer implements to load and save template sources, and worked notes for the two expected backings — committing to the consumer repo via a PR, and staging to a consumer-owned store before promotion.
• Preview: how preview.json sample data drives the in-editor preview, and how preview relates to the server-side Handlebars render (call out any behaviour differences rather than implying they are identical).
• How the editor output joins the documented lifecycle: editor -> content/email-templates/<key>/ -> PR -> CI -> post-kit-publish -> Blob. Link docs/guides/template-authoring.md and docs/guides/template-publishing.md instead of restating them.
• Access control guidance: the editor is an internal admin surface; the admin app owns authentication and authorisation, and the PostKit send credential must not be present in the admin browser bundle.

Add examples/admin-editor/

• README.md — what the example demonstrates and how to run it.
• A minimal React embedding plus an in-memory persistence adapter implementation, kept as small as the package allows.
• A node --test spec covering the adapter contract (load returns stored source; save round-trips; a save failure surfaces to the caller) without rendering a full browser environment if the package does not require it.
• package.json — private, not published, with a test script.

Update docs/README.md

• Add a row and structure entry for the guide.

Constraints

• Blocked until Epic: Build the reusable EmailBuilder.js admin editor package #5 is closed. The editor package does not exist yet; do not start this issue, and do not stub or invent the package's API. Document the API that ships.
• Do not build a full admin application — a minimal embedding plus an in-memory adapter is the target.
• The example must not include or require a PostKit send credential.
• Do not modify the editor package source from this issue; API gaps go back to Epic: Build the reusable EmailBuilder.js admin editor package #5 as comments.
• Public repository: placeholder tenants and example.com addresses only.
• If pnpm-workspace.yaml does not yet include an examples/* glob, add only that line.

Acceptance criteria

• docs/guides/editor-integration.md documents installation, embedding, the persistence adapter contract, and preview behaviour as actually implemented in @singleton-sd/post-kit-editor.
• The guide states clearly that the editor holds no PostKit send credential and that the admin app owns authentication/authorisation.
• The editor's place in the edit -> PR -> CI -> Blob -> send lifecycle is documented with links to the authoring and publishing guides.
• Any difference between in-editor preview and server-side render is stated explicitly.
• examples/admin-editor/ contains a minimal embedding, an in-memory persistence adapter, a passing spec, and a README.
• The example package is private and excluded from publication.
• pnpm -r --if-present run test, pnpm lint, and pnpm format:check pass.
• docs/README.md lists the new guide.

Agent implementation notes

Do not start until #5 is closed. When it is, read the shipped packages/post-kit-editor source, README, and specs first, and confirm the persistence adapter interface from the code before writing any of it down. Also read docs/guides/template-authoring.md and docs/guides/template-publishing.md (from the sibling documentation issues under #7) so the lifecycle description stays consistent. Branch: docs/55-editor-integration-guide.