feat: re-enable cronTool service after refactor completion

Re-enable cronTool service integration after completing the ToolResult
refactor (US-016). Removed all temporary disable comments and restored
full cron service lifecycle including start/stop operations.

Additional improvements:
- Add thread-safe access to onHeartbeatWithTools handler
- Fix channel parsing to handle user IDs with special characters
- Add error handling for state file loading failures

pkg/state/state.go

@@ -3,6 +3,7 @@ package state
 import (
 	"encoding/json"
 	"fmt"
+	"log"
 	"os"
 	"path/filepath"
 	"sync"
@@ -45,7 +46,10 @@ func NewManager(workspace string) *Manager {
 	}
 
 	// Load existing state if available
-	sm.load()
+	if err := sm.load(); err != nil {
+		// Log warning but continue with empty state
+		log.Printf("[WARN] state: failed to load state file, starting fresh: %v", err)
+	}
 
 	return sm
 }