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.

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 {