feat: implement background agents and async messaging system

[echobt]

Summary

This PR implements the complete background worker architecture and asynchronous messaging system for the CLI as specified in ORCHESTRATE/AGENT_1_BACKGROUND_AGENTS.md.

Changes

New Background Agent Module (cortex-agents/src/background/)

• executor.rs: BackgroundAgentManager for spawning and managing background tasks

  • Configurable max concurrent agents (default: 5)
  • Automatic timeout after 30 minutes
  • RAII cleanup with proper cancellation support
  • Event broadcasting for monitoring

• messaging.rs: Inter-agent async messaging system

  • AgentMailbox for per-agent message queues
  • MessageRouter for routing messages between agents
  • Support for notifications, requests/responses, data sharing, and status updates

• events.rs: Event system for background agents

  • AgentEvent enum for lifecycle events (Started, Progress, Completed, Failed, Cancelled)
  • NotificationManager for collecting and displaying notifications
  • Support for TodoUpdated events for progress tracking

TUI Enhancements (cortex-tui/)

• New /tasks command (aliases: /bg, /background) for viewing background tasks
• TasksView component for displaying task status, duration, and progress
• ModalType::Tasks for the background tasks modal

Tests

• 21 new unit tests covering:
  • Background agent spawning and cancellation
  • Max concurrent agent limits
  • Event subscription and notification
  • Inter-agent messaging
  • Notification management

Verification

• cargo check -p cortex-agents ✅
• cargo check -p cortex-tui ✅
• cargo fmt ✅
• cargo test -p cortex-agents -- background ✅ (21 tests pass)

Related

• Implements requirements from ORCHESTRATE/AGENT_1_BACKGROUND_AGENTS.md