Implement Session Persistence

[justin4957]

Description

Add persistence layer for conversation sessions to support multi-turn interactions.

Tasks

• Design session storage schema
• Implement file-based session storage
• Add session save/load functionality
• Support resuming previous sessions
• Add session listing and cleanup
• Implement session export (JSON format)
• Add encryption for sensitive data
• Add tests for persistence layer

Files

• lib/multi_agent_coder/session/manager.ex
• lib/multi_agent_coder/session/storage.ex (new)
• test/multi_agent_coder/session/*_test.exs

Dependencies

May use :ets or file storage