Create standalone pod_executor package

Created a standalone code execution package independent of MCP-Forge:

Structure:
- pod_executor/security/ - Resource limits, audit protocols, validation
- pod_executor/containers/ - Podman client and container management
- pod_executor/simple/ - Stateless code executor
- pod_executor/jupyter/ - Stateful Jupyter backend with sessions

Key changes:
- Removed ForgeConfig dependency - all parameters explicit
- Audit logger now a protocol with NullAuditLogger/SimpleFileAuditLogger
- Validator now a protocol with NoOpValidator/BasicValidator
- All imports updated to pod_executor namespace
- Audit calls use simple strings instead of enums

Benefits:
- Standalone package usable without MCP-Forge
- Clear separation between execution engine and MCP protocol
- Easier testing and development
- Reusable in other projects

src/pod_executor/simple/__init__.py

@@ -0,0 +1,8 @@
+"""Simple stateless code executor."""
+
+from pod_executor.simple.executor import CodeExecutor, ExecutionResult
+
+__all__ = [
+    "CodeExecutor",
+    "ExecutionResult",
+]