Implement OverlayFS support for fcctl-core #668

Summary

Implement OverlayFS support for session-specific packages in Firecracker VMs.

Context

PR #426 (feat(rlm): RLM orchestration with MCP tools) requires OverlayFS support for isolation of package installations per session.

Requirements

The implementation should provide:

1. OverlayFS mount management for VM root filesystem
2. Session-specific overlay layers
3. Package installation isolation
4. Cleanup of overlay layers on session end

Related

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