Implement macOS Native Agent Backend PoC

## User Story

As a Backend.AI developer, I want to run Agent backend components natively on macOS so that I can develop and test without Docker Desktop overhead.

## Implementation Details

- Extract backend components from existing Agent code
- Implement integration layer for macOS native container runtime
- Create standalone execution mode for backend processes
- Ensure compatibility with existing Agent architecture

## Acceptance Criteria

- Backend components can be executed independently on macOS
- Container runtime integration works with selected native runtime
- Basic agent operations function correctly in PoC mode
- Documentation for setup and usage is provided

## Technical Notes

- Focus on extracting minimal viable backend functionality
- Defer full feature parity to future iterations

JIRA Issue: BA-4044