Implement output streaming for fcctl-core #670

Summary

Implement output streaming for VM execution results.

Context

PR #426 (feat(rlm): RLM orchestration with MCP tools) requires output streaming for real-time execution feedback.

Requirements

The implementation should provide:

1. Streaming of stdout/stderr from VM execution
2. Real-time updates to clients
3. Buffer management for large outputs
4. Integration with SSH execution

Related

Blocked by: #664 (Create fcctl-core crate)