chore: Add agentic coding tooling

.crosslink/rules/java.md

@@ -0,0 +1,42 @@
+### Java Best Practices
+
+#### Code Style
+- Follow Google Java Style Guide or project conventions
+- Use meaningful variable and method names
+- Keep methods short (< 30 lines)
+- Prefer composition over inheritance
+
+#### Error Handling
+```java
+// GOOD: Specific exceptions with context
+public Config readConfig(Path path) throws ConfigException {
+    try {
+        String content = Files.readString(path);
+        return objectMapper.readValue(content, Config.class);
+    } catch (IOException e) {
+        throw new ConfigException("Failed to read config: " + path, e);
+    } catch (JsonProcessingException e) {
+        throw new ConfigException("Invalid JSON in config: " + path, e);
+    }
+}
+
+// BAD: Catching generic Exception
+public Config readConfig(Path path) {
+    try {
+        return objectMapper.readValue(Files.readString(path), Config.class);
+    } catch (Exception e) {
+        return null;  // Swallowing error
+    }
+}
+```
+
+#### Security
+- Use PreparedStatement for SQL (never string concatenation)
+- Validate all user input
+- Use secure random (SecureRandom) for security-sensitive operations
+- Never log sensitive data (passwords, tokens)
+
+#### Testing
+- Use JUnit 5 for unit tests
+- Use Mockito for mocking dependencies
+- Aim for high coverage on business logic