feat(plugin-kit): scaffold @punktfunk/plugin-kit + phase-0 spikes

Two spikes validating the riskiest seams of the plugin-kit design before
implementation:

- HttpApi (effect/unstable/httpapi) served on Bun behind the SDK's
  servePluginUi using ONLY effect-core layers (Etag.layerWeak, Path.layer,
  HttpPlatform.layer + FileSystem.layerNoop) — no platform package. Auth,
  schema validation, and static fallthrough all verified end-to-end.
- Browser client prefix strategy for the console proxy: both
  transformClient+prependUrl AND baseUrl preserve the /plugin-ui/<id>
  path prefix. Nested Schema.withDecodingDefaultKey defaults confirmed
  (Effect-valued defaults; encodingStrategy "omit" restores raw shape
  on encode).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugin-kit/src/index.ts

@@ -0,0 +1,5 @@
+// @punktfunk/plugin-kit — Effect-based framework for punktfunk plugins.
+// Modules land here as they are implemented; the spikes in test/ validate the
+// riskiest seams (HttpApi behind servePluginUi, client prefix handling) first.
+
+export {}