fix: correct SDK import and clear stale agent UI on stop

Changes:
- Revert incorrect import from claude_code_sdk to claude_agent_sdk in agent.py
  (PR #50 introduced an undocumented change to a deprecated package)
- Clear activeAgents and recentActivity in useWebSocket when agent stops
  to prevent stale UI state

The claude_code_sdk package is deprecated (last updated Sep 2025) while
claude_agent_sdk is the active, maintained package. The import change in
PR #50 was undocumented and would have caused ImportError since only
claude-agent-sdk is specified in requirements.txt.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent.py

@@ -14,7 +14,7 @@ from pathlib import Path
 from typing import Optional
 from zoneinfo import ZoneInfo
 
-from claude_code_sdk import ClaudeSDKClient
+from claude_agent_sdk import ClaudeSDKClient
 
 # Fix Windows console encoding for Unicode characters (emoji, etc.)
 # Without this, print() crashes when Claude outputs emoji like ✅

ui/src/hooks/useWebSocket.ts

@@ -112,6 +112,8 @@ export function useProjectWebSocket(projectName: string | null) {
               setState(prev => ({
                 ...prev,
                 agentStatus: message.status,
+                // Clear active agents when process stops to prevent stale UI
+                ...(message.status === 'stopped' && { activeAgents: [], recentActivity: [] }),
               }))
               break