Minimal example integration and end-to-end component tests

[patoperpetua]

Parent: #5
Depends on: #46, #47

Goal

Deliver a minimal, executable example integration of @singleton-sd/post-kit-editor plus end-to-end component tests that exercise the full editing flow. This proves the epic's acceptance criteria — a consumer can install the package, load existing template sources, edit them visually, preview, validate, and save through consumer-supplied callbacks — without building a demo application.

Scope

Minimal example

• Path: packages/post-kit-editor/examples/minimal/
• Content: a single small React entry file plus a short README showing the complete integration:

<EmailTemplateEditor
  template={template}
  availableVariables={variables}
  onSave={async (files) => {
    // Consumer commits files.templateJson / metadataJson / previewJson
    // to its own repository (e.g. via its own server endpoint).
  }}
  onSendTest={async (files, recipient) => {
    // Consumer POSTs to its own trusted server endpoint,
    // which uses @singleton-sd/post-kit-client server-side.
  }}
/>

• Include a sample template directory (template.json, metadata.json, preview.json) with synthetic values that the example loads.
• The example is documentation-grade: readable in under a minute, no routing, no application shell, no styling framework, no build pipeline of its own beyond what the package already has.
• Exclude examples/ from the published files array and from the package build output.

End-to-end component tests

Add src/e2e.spec.tsx (or an equivalently named suite) driving EmailTemplateEditor through the full flow with the same fixtures:

• Load a template source triple and assert the canvas, metadata, variables, and preview panels render populated.
• Edit metadata and preview data; assert changes reach the working state.
• Introduce an undeclared variable; assert the validation summary reports it and Save becomes disabled.
• Fix the issue; assert Save re-enables.
• Click Save; assert onSave receives serialised file contents that round-trip back to the edited state without information loss.
• Provide onSendTest; assert the control appears, validates the recipient, and invokes the callback.
• Assert no network request and no filesystem access occurs during the entire flow.

Documentation

• Update packages/post-kit-editor/README.md with the complete usage example, the props table for the finalised public API, and a "what this package does not do" section (no persistence, no sending, no credentials).
• Link the example directory from the README.

Constraints

• Build an example, not a demo application — no dashboard, no multi-page shell, no mock backend server.
• The example must not include any credential, token, endpoint, or customer-identifying value; sample data must be synthetic and generic.
• examples/ must not be published to npm and must not be part of the package's dist output.
• Do not add a browser-based test runner (Playwright, Cypress, jsdom-heavy stacks) — use the node --test setup established by the scaffold.
• Do not change the public API in this issue except to correct genuine defects found while writing the example; substantive API changes belong in the issue that introduced the surface.

Acceptance criteria

• packages/post-kit-editor/examples/minimal/ contains a runnable single-file integration plus a synthetic sample template directory and a short README.
• examples/ is excluded from the published package files and from build output.
• An end-to-end component suite drives load → edit → validate → save with the package's public API only.
• The suite asserts onSave receives serialised contents that round-trip losslessly to the edited state.
• The suite asserts validation errors disable Save and that fixing them re-enables it.
• The suite asserts the send-test control appears only with onSendTest, validates the recipient, and invokes the callback.
• The suite asserts no network or filesystem access occurs.
• README documents the full usage example, the props table, and the package's non-responsibilities.
• No secrets, endpoints, or customer-identifying values appear anywhere in the example or fixtures.
• pnpm -r --if-present run test, pnpm build, pnpm lint, and pnpm format:check all pass.

Agent implementation notes

Read the finalised prop surface from #46 (save/send-test) and #47 (validation, loading/error states) before writing the example — the example must use the shipped API verbatim, not an idealised one. Reuse the fixtures added in the canvas/serialization issue rather than creating new ones. Read AGENTS.md "Public-repo safety" before adding any sample values. Branch: feat/<issue-number>-editor-example-and-e2e-tests.