Migrate buildSrc to conventions directory

- Migrate all convention plugins from buildSrc/ to conventions/
- Remove buildSrc directory to align with upstream OpenTelemetry
- Update settings.gradle.kts to use includeBuild("conventions")
- Update .gitignore to ignore conventions/.gradle and .kotlin
- Add Gradle alignment plan documentation

This follows the upstream pattern of separating pure build logic
(conventions/) from compiled plugins (gradle-plugins/ - to be added
in future PR). Conventions are included as a composite build.

Author: trask

.gitignore

@@ -1,8 +1,8 @@
 # Gradle
 /.gradle/
-/buildSrc/.gradle/
+/conventions/.gradle/
+/conventions/.kotlin/
 /perf-tests/.gradle/
-/buildSrc/.kotlin/
 build/
 
 # Intellij

buildSrc/build.gradle.kts conventions/build.gradle.ktsbuildSrc/build.gradle.kts renamed to conventions/build.gradle.kts

@@ -1,5 +1,4 @@
 plugins {
-  `java-gradle-plugin`
   `kotlin-dsl`
   // When updating, update below in dependencies too
   id("com.diffplug.spotless") version "8.2.0"
@@ -22,10 +21,6 @@ repositories {
   gradlePluginPortal()
 }
 
-tasks.withType<Test>().configureEach {
-  useJUnitPlatform()
-}
-
 dependencies {
   implementation(gradleApi())
 

conventions/settings.gradle.kts

@@ -0,0 +1 @@
+rootProject.name = "conventions"