Add export/import for Nexus configuration

[FatBy]

Description

Users should be able to export a Nexus configuration (including SOP, bound skills, model assignment) as a JSON/YAML file and share it with others.

Current Behavior

• Nexus configs exist as NEXUS.md files in the nexuses/ directory
• No UI for export/import
• Sharing requires manual file copying

Expected Behavior

• "Export" button on Nexus detail panel downloads a .nexus.json file
• "Import Nexus" option in World View uploads a .nexus.json to create a new Nexus
• Exported file includes: name, SOP, bound skills list, visual_dna, model preferences

Notes

This enables a community ecosystem where users can share specialized Nexus configurations.