Add support for Java language scanning

[dockfixlabs]

Description

Currently AgentGuard scans Python, JS, TS, and other languages with regex-based rules. Java support would expand coverage.

What needs to be done

• Add .java to SCANABLE_EXTENSIONS in scanner.py
• Test existing rules against Java code samples
• Add Java-specific patterns where needed (e.g., Runtime.getRuntime().exec())

Files to modify

• �gentguard/scanner.py — add .java to extensions
• ests/test_scanner.py — add Java test cases
• �gentguard/rules/ — add Java-specific patterns if needed