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.

server/build.gradle.kts

@@ -0,0 +1,30 @@
+plugins {
+    alias(libs.plugins.kotlin.multiplatform)
+    alias(libs.plugins.kotlin.serialization)
+}
+
+kotlin {
+    jvmToolchain(21)
+    jvm()
+
+    sourceSets {
+        commonMain.dependencies {
+            api(project(":api"))
+            implementation(libs.kotlinx.coroutines.core)
+            implementation(libs.kotlinx.serialization.json)
+            implementation(libs.ktor.server.core)
+            implementation(libs.ktor.server.sse)
+        }
+        jvmMain.dependencies {
+            implementation(libs.ktor.server.netty)
+        }
+        jvmTest.dependencies {
+            implementation(kotlin("test"))
+            implementation(libs.kotlinx.coroutines.test)
+            implementation(libs.ktor.server.test.host)
+            implementation(libs.ktor.client.core)
+            implementation(libs.ktor.client.cio)
+            implementation(project(":client"))
+        }
+    }
+}