Resolves#87 and #9 by implementing a robust three-tier process
termination approach:
1. ProcessRegistry kill - primary method using run_id tracking
2. ClaudeProcessState kill - fallback via stored process handle
3. System kill command - last resort using PID and OS commands
Key improvements:
- Enhanced logging throughout termination flow for better debugging
- Graceful fallback between termination methods
- Proper UI state management even when backend termination fails
- Track run_id in AgentExecution component for targeted process killing
- Comprehensive error handling with user-friendly feedback
- Consistent event emission for UI synchronization
This ensures agents can be properly stopped without requiring
application restart, addressing the core issue where STOP
requests were ignored and processes continued running.
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