feat: add experience-toolbar starter example [EXT-7365]

[Jared Jolton (jjolton-contentful)]

EXT-7365

Summary

• Adds a Create Contentful App example at examples/experience-toolbar for the new ExO toolbar App SDK location (LOCATION_EXPERIENCE_TOOLBAR), scaffoldable via npx create-contentful-app --example experience-toolbar.
• Typed with ExperienceEditorToolbarAppSDK from @contentful/app-sdk@4.58.0 (first example to pin 4.58.0). Demonstrates every pattern in the ticket AC:
  • location detection — sdk.location.is(locations.LOCATION_EXPERIENCE_TOOLBAR) in App.tsx
  • reading sdk.exo.context to tell experience vs. fragment apart (+ onContextChanged)
  • subscribing to sdk.exo.experience.selection.onChange()
  • resolving the selected node via sdk.exo.experience.getNode(nodeId) and reading its properties
  • reacting to sdk.exo.onUiModeChanged() (form/visual), with graceful form-mode degradation
• React-only; no Data Assembly or save()/publish() (per ticket excluded scope). Mirrors examples/typescript conventions (Vite + Vitest + Forma 36).

Two deviations from the AC (intentional — flagging for review)

1. --example, not --template. The AC says --template experience-toolbar, but the CLI scaffolds repo examples via --example <name> (resolved from apps/examples/ via the GitHub API on master). --template is a fixed allowlist of framework starters (IGNORED_EXAMPLE_FOLDERS). Delivered as an example, matching how editor-assignment, page-location, etc. ship. The ticket hedges with "(or equivalent)".
2. No app-definition.json. No example in the repo ships a manifest, and the toolbar location is not an EditorInterface target state — visibility is determined solely by whether the location is registered on the app definition. Documented registration via create-app-definition / add-locations in the README and a ConfigScreen note instead of inventing a manifest the CLI would strip.

Verification status

• ✅ Type-verified: tsc --noEmit clean against the published 4.58.0 types.
• ✅ vite build clean.
• ✅ 7/7 Vitest specs pass (mocked SDK drives selection / ui-mode / context subscriptions).
• ⚠️ Not runtime-verified inside a live ExO editor — the host renderer serving sdk.exo is still rolling out (EXT-7363/EXT-7364). The README states this explicitly. API shapes match the published types exactly.

Test plan

• npm install && npm run build in examples/experience-toolbar succeeds
• npm run test:ci passes
• npx create-contentful-app --example experience-toolbar scaffolds a working project (once merged to master, since the CLI reads examples from the GitHub API)
• (When host renderer lands) install into a space, register the experience-toolbar location, open an experience, and confirm context/selection/ui-mode/properties render live

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