feat: add bgrun extension for background task management

[kostyay]

• Add /bgrun command to start background tasks via tmux with auto-derived window names (e.g. npm run dev → npm-dev)
• Implement /bgtasks command with interactive task manager TUI for listing, viewing output, and killing tasks
• Add bgrun tool for LLM to start/list/capture/kill background tasks programmatically
• Emit bgrun:stats event to provide background task counts for status-bar integration
• Auto-prune dead tasks on sync, kill all tasks when starting a new session (/new), and prompt on shutdown with 5-second confirmation
• Update status-bar to display background task counts alongside ticket stats
• Add control-channel.ts library for UDP-based inter-process communication
• Add comprehensive test suite covering task naming, tmux integration, and interactive task manager flows
• Document bgrun and op-timer extensions in README and CHANGELOG