feat(mcp): switch to YAML serialization for tool outputs

- Replaced serde_json with serde_yaml in all handler endpoints.

- This drastically reduces token consumption for large list/graph responses by removing JSON braces and quotes.

server/src/handlers/graph.rs

@@ -542,7 +542,7 @@ impl McpTool for ReadGraphHandler {
                 })
             };
 
-            let mut result_json = serde_json::to_string(&serialized)?;
+            let mut result_json = serde_yaml::to_string(&serialized)?;
 
             if let Some(max_tok) = max_tokens {
                 let max_chars = max_tok * 4;
@@ -579,7 +579,7 @@ impl McpTool for ReadGraphHandler {
                                 obj.insert("has_more".to_string(), serde_json::Value::Bool(true));
                             }
 
-                        if let Ok(trunc_str) = serde_json::to_string(&truncated_val)
+                        if let Ok(trunc_str) = serde_yaml::to_string(&truncated_val)
                             && (trunc_str.len() <= max_chars || keep_count == 1) {
                                 result_json = trunc_str;
                                 break;
@@ -638,7 +638,7 @@ impl McpTool for SearchNodesHandler {
                     }
                 }
             }
-            Ok::<String, crate::error::AppError>(serde_json::to_string(&matched_entities)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&matched_entities)?)
         })?;
         Ok(data)
     }
@@ -679,7 +679,7 @@ impl McpTool for OpenNodesHandler {
                     result.entities.insert(name, e);
                 }
             }
-            Ok::<String, crate::error::AppError>(serde_json::to_string(&result)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&result)?)
         })?;
         Ok(data)
     }
@@ -954,7 +954,7 @@ impl McpTool for FindOrphansHandler {
                 .cloned()
                 .collect::<Vec<String>>()
         });
-        let data = Ok::<String, crate::error::AppError>(serde_json::to_string(&orphans)?)?;
+        let data = Ok::<String, crate::error::AppError>(serde_yaml::to_string(&orphans)?)?;
         Ok(data)
     }
 }
@@ -1047,7 +1047,7 @@ impl McpTool for GetSubgraphHandler {
             "relations": matched_relations,
         });
 
-        Ok(serde_json::to_string_pretty(&result)?)
+        Ok(serde_yaml::to_string(&result)?)
     }
 }
 
@@ -1146,7 +1146,7 @@ impl McpTool for SweepGraphHealthHandler {
             "health_score": if orphans.is_empty() && duplicates.is_empty() && stale_entities.is_empty() { "100%" } else { "Needs Maintenance" }
         });
 
-        Ok(serde_json::to_string_pretty(&report)?)
+        Ok(serde_yaml::to_string(&report)?)
     }
 }
 
@@ -1203,7 +1203,7 @@ impl McpTool for ResolveStaleSymbolsHandler {
             "status": "success"
         });
 
-        Ok(serde_json::to_string_pretty(&report)?)
+        Ok(serde_yaml::to_string(&report)?)
     }
 }