Remove all Gradle references, migrate comments to Bazel (williamfiset#1270)

- Delete build/ directory from tracking (193 generated files) and
  add it to .gitignore
- Delete .github/workflows/gradle.yml CI workflow
- Remove Gradle ecosystem from .github/dependabot.yml
- Replace ./gradlew run comments with bazel run in 22 source files
- Replace gradle test comments with bazel test in 14 test files

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: williamfiset

.github/dependabot.yml

@@ -4,7 +4,3 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "weekly"
-  - package-ecosystem: "gradle"
-    directory: "/"
-    schedule:
-      interval: "weekly"

.github/workflows/gradle.yml

@@ -1,5 +1,8 @@
 *.class
 
+# Gradle build outputs
+build/
+
 # Bazel build outputs
 bazel-*
 /bazel-bin