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.

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" }