feat(rules): add Java language rules (#645)

Adds Java language rules (coding-style, hooks, patterns, security, testing) following the established language rule conventions.

rules/java/hooks.md

@@ -0,0 +1,18 @@
+---
+paths:
+  - "**/*.java"
+  - "**/pom.xml"
+  - "**/build.gradle"
+  - "**/build.gradle.kts"
+---
+# Java Hooks
+
+> This file extends [common/hooks.md](../common/hooks.md) with Java-specific content.
+
+## PostToolUse Hooks
+
+Configure in `~/.claude/settings.json`:
+
+- **google-java-format**: Auto-format `.java` files after edit
+- **checkstyle**: Run style checks after editing Java files
+- **./mvnw compile** or **./gradlew compileJava**: Verify compilation after changes