feat: wire all static check plugins into build.gradle.kts and CI pipeline

- Apply 6 new lint plugins in root build.gradle.kts
- Add check_pr_hygiene CI job for shell-based PR checks
- All new checks are advisory (warnings only) except checkEmptyInstrumentations

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: bm1549

.gitlab-ci.yml

@@ -468,6 +468,17 @@ check_build_src:
   variables:
     GRADLE_TARGET: ":buildSrc:build"
 
+check_pr_hygiene:
+  extends: .gradle_build
+  needs: []
+  stage: tests
+  script:
+    - bash scripts/check-ci-debug-flags.sh || true
+    - bash scripts/check-extraneous-files.sh || true
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_COMMIT_BRANCH != "master"
+
 check_base:
   extends: .check_job
   variables:

build.gradle.kts

@@ -9,6 +9,12 @@ plugins {
   id("dd-trace-java.tracer-version")
   id("dd-trace-java.dump-hanged-test")
   id("dd-trace-java.config-inversion-linter")
+  id("dd-trace-java.empty-instrumentation-linter")
+  id("dd-trace-java.unnecessary-else-linter")
+  id("dd-trace-java.naming-convention-linter")
+  id("dd-trace-java.javadoc-linter")
+  id("dd-trace-java.copy-paste-detector")
+  id("dd-trace-java.assertj-preference-linter")
   id("dd-trace-java.ci-jobs")
 
   id("com.diffplug.spotless") version "8.2.1"