Refactor DESIGN.md to enhance data flow visualization and clarity

- Replaced the previous text-based data flow representation with a flowchart using Mermaid syntax for improved readability and understanding.
- Streamlined the depiction of data sources and their relationships, clarifying the flow from content module to LLM input.
- Updated the documentation to ensure consistency with the new visual format, aiding in the comprehension of the search module's architecture.

agent/search/DESIGN.md

@@ -1193,38 +1193,22 @@ type Reference struct {
 
 **Data Flow:**
 
-```
+```mermaid
-┌─────────────────┐    ┌─────────────────┐    ┌─────────────────┐
+flowchart TD
-│ Content Module  │    │   Hook Search   │    │   Auto Search   │
+    subgraph Sources ["Data Sources"]
-│ (db:xxx kb:xxx) │    │ ctx.search.*()  │    │ (assistant cfg) │
+        CM["Content Module<br/>(db:xxx kb:xxx)"]
-└────────┬────────┘    └────────┬────────┘    └────────┬────────┘
+        HS["Hook Search<br/>ctx.search.*()"]
-         │                      │                      │
+        AS["Auto Search<br/>(assistant config)"]
-         │ source="user"        │ source="hook"        │ source="auto"
+    end
-         │ weight=1.0           │ weight=0.8           │ weight=0.6
+
-         │                      │                      │
+    CM -->|"source=user<br/>weight=1.0"| REF
-         └──────────────────────┼──────────────────────┘
+    HS -->|"source=hook<br/>weight=0.8"| REF
-                                │
+    AS -->|"source=auto<br/>weight=0.6"| REF
-                                ▼
+
-                    ┌───────────────────────┐
+    REF["[]Reference<br/>(Unified Structure)"]
-                    │     []Reference       │
+    REF --> MERGE["Merge & Deduplicate<br/>Rerank by score × weight"]
-                    │  (Unified Structure)  │
+    MERGE --> BUILD["Build &lt;references&gt; XML"]
-                    └───────────┬───────────┘
+    BUILD --> LLM["LLM Input"]
-                                │
-                                ▼
-                    ┌───────────────────────┐
-                    │  Merge & Deduplicate  │
-                    │  Rerank by score*wt   │
-                    └───────────┬───────────┘
-                                │
-                                ▼
-                    ┌─────────────────────────────┐
-                    │  Build <references> XML     │
-                    └───────────┬─────────────────┘
-                                │
-                                ▼
-                    ┌───────────────────────┐
-                    │      LLM Input        │
-                    └───────────────────────┘
 ```
 
 **LLM References Format:**