feat: AG-UI as in-process KMP API with Ktor client/server transports

Model the AG-UI protocol as an in-process API of Java-style interfaces and
classes, mirroring the official AG-UI Java SDK, and build it as a Kotlin
Multiplatform library with two Ktor transport modules.

Modules:
- api: BaseEvent sealed hierarchy (all AG-UI event types), messages, state,
  tool, context, RunAgentInput/Parameters, Agent/AgentSubscriber/
  EventDispatcher, AguiJson (kotlinx.serialization, "type" discriminator),
  plus a SerializersModule registering every event subtype polymorphically.
- client: HttpAgent over Ktor SSE (POST RunAgentInput -> stream BaseEvent),
  implementing the Agent interface.
- server: Application.agui()/Route.aguiAgent() exposing an Agent behind a
  POST + SSE endpoint via SSEServerContent, plus a runnable sample.

Tests: api serialization round-trips, client SSE decode against a real
test-host server, server endpoint integration.

build.gradle.kts

@@ -0,0 +1,9 @@
+plugins {
+    alias(libs.plugins.kotlin.multiplatform) apply false
+    alias(libs.plugins.kotlin.serialization) apply false
+}
+
+allprojects {
+    group = "com.agui"
+    version = "0.1.0-SNAPSHOT"
+}