Bring up :proto protocol + :server (Ktor) + :client (HTTP) modules; wire :server into standalone with EchoProtoAgent

Major additions:

* :proto (KMP submodule) — in-house stateful protocol replacing AG-UI.
  Agent owns conversation transcript; Conversation.events(after) is a live,
  replay-free stream; backfill via Conversation.getMessages(after, offset, limit).
  Each Event carries an Instant date for client-side resume tracking.
  Sealed hierarchies (Content/Message/Event/AgentEvent) annotated @Serializable
  with snake_case @SerialName JSON discriminators so the wire format is
  decoupled from Kotlin class names.

* :server (JVM, Ktor 3.1.3) — REST+SSE facade for Agent.
  Public entry: Route.agentikAgent(agent, path = "/agentik").
  Endpoints: create/list/get/patch/delete conversations, POST messages (202),
  POST interrupt, GET messages, GET conversation events (SSE),
  GET agent events (SSE), GET /health. Custom Instant serializer for
  kotlin.time.Instant registered contextually on agentikJson (ISO-8601,
  ignoreUnknownKeys=true, explicitNulls=false).

* :client (JVM, Ktor HTTP Client + CIO) — mirror of :server returning
  a pw.binom.agentik.proto.Agent backed by HTTP calls. Custom SSE parser
  since ktor-client-sse is not on the 3.1.3 client classpath.

* standalone — EchoProtoAgent (in-memory Agent for :proto), EchoAgent
  (existing AG-UI echo), both mounted on the same Netty embedded server
  on port 8080 (/agui and /agentik); A2A stays on its own CIO engine on
  8081. EchoProtoAgent smoke-tested end-to-end against :server: all 11
  endpoints, including live SSE delivery of StartResponse/AppendText/End
  event triplets and Agent-level Created/Deleted events.

Design notes pinned in:
* agentik/IRC-QUESTIONS.md — closed 13-item checklist for the upcoming
  :irc-server transport (channel = conversation, CTCP for structural
  events, draft/chathistory for backfill, ImageStore side-channel, etc).
* docs/ARCHITECTURE.md — overall layout snapshot.

proto/build.gradle.kts

@@ -0,0 +1,30 @@
+plugins {
+    alias(libs.plugins.kotlin.multiplatform)
+    alias(libs.plugins.kotlin.serialization)
+}
+
+kotlin {
+    jvmToolchain(21)
+
+    // "Все возможные цели сборки": jvm + весь натив. Зеркалит набор AG-UI api.
+    jvm()
+    macosX64()
+    macosArm64()
+    iosX64()
+    iosArm64()
+    iosSimulatorArm64()
+    linuxX64()
+    linuxArm64()
+    mingwX64()
+
+    sourceSets {
+        commonMain.dependencies {
+            api(libs.kotlinx.coroutines.core)
+            api(libs.kotlinx.datetime)
+            api(libs.kotlinx.serialization.core)
+        }
+        commonTest.dependencies {
+            implementation(kotlin("test"))
+        }
+    }
+}