docs: Add Use Case Diagramdocs: Add Use Case Diagramdocs: Add Use Case Diagramdocs: Add Use Case Diagramdocs: add use case diagram

design-assets/use_case_diagram.md

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+# Sentinel Core System - Use Case Diagram
+
+```mermaid
+flowchart LR
+    %% Actors
+    User("👤<br>User<br>(CLI / Dev)")
+    APIClient("👤<br>API Client<br>(External UI)")
+    LLM("👤<br>LLM Provider<br>(e.g. Ollama)")
+
+    subgraph System ["Sentinel Core System"]
+        direction TB
+
+        subgraph File_Management_Operations ["File Management Operations"]
+            direction TB
+            ScanDir(["Scan Directory"]):::green
+            PrevFiles(["Preview Files"]):::green
+            CleanPC(["Clean PC"]):::green
+        end
+
+        subgraph AI_Planning_Module ["AI Planning Module"]
+            direction TB
+            GenPlan(["Generate Org. Plan"]):::purple
+            AskAss(["Ask Assistant"]):::purple
+        end
+
+        subgraph Execution_Reversibility ["Execution & Reversibility"]
+            direction TB
+            ExecPlan(["Execute Plan (Apply)"]):::orange
+            UndoExec(["Undo Execution"]):::orange
+        end
+
+        subgraph System_Monitoring ["System Monitoring"]
+            direction TB
+            StreamEvents(["Stream Real-time Events"]):::blue
+        end
+    end
+
+    %% Associations
+    User --- ScanDir
+    User --- PrevFiles
+    User --- CleanPC
+    User --- GenPlan
+    User --- AskAss
+    User --- ExecPlan
+    User --- UndoExec
+
+    APIClient --- ScanDir
+    APIClient --- GenPlan
+    APIClient --- ExecPlan
+    APIClient --- StreamEvents
+
+    GenPlan --- LLM
+    AskAss --- LLM
+
+    %% Dependencies
+    CleanPC -. "«include»" .-> ScanDir
+    GenPlan -. "«extend»" .-> CleanPC
+    UndoExec -. "«extend»" .-> ExecPlan
+
+    classDef green fill:#e6ffe6,stroke:#009900,stroke-width:2px;
+    classDef purple fill:#e6e6ff,stroke:#6600cc,stroke-width:2px;
+    classDef orange fill:#ffeedd,stroke:#cc6600,stroke-width:2px;
+    classDef blue fill:#cce6ff,stroke:#0066cc,stroke-width:2px;
+```