proto: drop dead kotlinx-datetime dep, keep only kotlin.time.Instant

Punkt 7 of NATIVE-COMPATIBILITY.md. Proto sources were already on
kotlin.time.Instant (migrated earlier); the kotlinx-datetime api dep
in :proto/build.gradle.kts and the implementation dep in
:standalone/build.gradle.kts were dead weight.

Dropped:
  - api(libs.kotlinx.datetime) from proto/build.gradle.kts
  - implementation(libs.kotlinx.datetime) from standalone/build.gradle.kts
  - [versions] kotlinx-datetime and [libraries] kotlinx-datetime from
    gradle/libs.versions.toml

Verification: ./gradlew :server:assemble (all 9 native targets) +
./gradlew :standalone:jvmTest (44/44 green, 0 failures) — both pass.
The kotlinx-datetime typealias was deprecated in 0.8.0 (memory #3708);
now nothing in agentik pulls the library.

NATIVE-COMPATIBILITY.md

@@ -55,12 +55,16 @@
 
 ## Блок 4. Наш `:proto` модуль
 
-- 7. [ ] **`:proto`: заменить `kotlinx.datetime.Instant` на `kotlin.time.Instant`.**
-  Per Memory #3708 — typealias `kotlinx.datetime.Instant` deprecated в
-  kotlinx-datetime 0.8.0, нужно перейти на `kotlin.time.Instant`
-  (он же в commonMain). После: дропнуть `api(libs.kotlinx.datetime)` из
-  commonMain, и из `libs.versions.toml`. Объём: замена импорта + типов +
-  возможно `InstantSerializer` в `:server/src/main/.../Serialization.kt`.
+- 7. [x] **`:proto`: убрать `kotlinx-datetime` api-dep, оставить `kotlin.time.Instant`.**
+  Источник уже был на `kotlin.time.Instant` (мигрирован ранее), но
+  `kotlinx-datetime` оставался в gradle как мёртвая `api`-зависимость.
+  Удалено:
+  - `api(libs.kotlinx.datetime)` из `proto/build.gradle.kts`
+  - `implementation(libs.kotlinx.datetime)` из `standalone/build.gradle.kts`
+  - `[versions] kotlinx-datetime` и `[libraries] kotlinx-datetime` из
+    `gradle/libs.versions.toml`
+  `kotlin.time.Instant` доступен во всех KMP-целях без opt-in.
+  `./gradlew :server:assemble` + `:standalone:jvmTest` 44/44 — зелёные.
 
 ## Блок 5. SQLDelight — нативный драйвер
 

gradle/libs.versions.toml

@@ -2,7 +2,6 @@
 kotlin = "2.4.20"
 kotlinx-serialization = "1.11.0"
 kotlinx-coroutines = "1.11.0"
-kotlinx-datetime = "0.8.0"
 ktor = "3.1.3"
 agui = "0.1.0"
 a2a = "1.0.0-SNAPSHOT"
@@ -58,4 +57,3 @@ kotlin-test = { module = "org.jetbrains.kotlin:kotlin-test", version.ref = "kotl
 kotlinx-coroutines-core = { module = "org.jetbrains.kotlinx:kotlinx-coroutines-core", version.ref = "kotlinx-coroutines" }
 kotlinx-serialization-core = { module = "org.jetbrains.kotlinx:kotlinx-serialization-core", version.ref = "kotlinx-serialization" }
 kotlinx-serialization-json = { module = "org.jetbrains.kotlinx:kotlinx-serialization-json", version.ref = "kotlinx-serialization" }
-kotlinx-datetime = { module = "org.jetbrains.kotlinx:kotlinx-datetime", version.ref = "kotlinx-datetime" }

proto/build.gradle.kts

@@ -20,7 +20,6 @@ kotlin {
     sourceSets {
         commonMain.dependencies {
             api(libs.kotlinx.coroutines.core)
-            api(libs.kotlinx.datetime)
             api(libs.kotlinx.serialization.core)
         }
         commonTest.dependencies {

standalone/build.gradle.kts

@@ -28,7 +28,6 @@ kotlin {
             // Commons
             implementation(libs.kotlinx.coroutines.core)
             implementation(libs.kotlinx.serialization.json)
-            implementation(libs.kotlinx.datetime)
 
             // litert-kmp: контракт (commonMain)
             api(libs.litert.api)