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/meta.rs

@@ -142,7 +142,7 @@ impl McpTool for SearchErrorFixesHandler {
                     })
                     .collect();
 
-                Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&suggestions)?)
+                Ok::<String, crate::error::AppError>(serde_yaml::to_string(&suggestions)?)
             })?;
             return Ok(data);
         }
@@ -169,7 +169,7 @@ impl McpTool for SearchErrorFixesHandler {
                     }
                 })
                 .collect();
-            Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&filtered)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
         })?;
         Ok(data)
     }
@@ -358,7 +358,7 @@ impl McpTool for QueryRecentChangesHandler {
                 .take(limit)
                 .cloned()
                 .collect();
-            Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+            Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
         })?;
         Ok(data)
     }
@@ -575,7 +575,7 @@ impl McpTool for DecisionsHandler {
                         .collect();
 
                     if include_body {
-                        Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                     } else {
                         let compact: Vec<_> = filtered
                             .iter()
@@ -592,7 +592,7 @@ impl McpTool for DecisionsHandler {
                                 })
                             })
                             .collect();
-                        Ok::<String, crate::error::AppError>(serde_json::to_string(&compact)?)
+                        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&compact)?)
                     }
                 })?;
                 Ok(data)
@@ -738,7 +738,7 @@ impl McpTool for TechDebtHandler {
                             }),
                         })
                         .collect();
-                    let mut json_str = serde_json::to_string(&filtered)?;
+                    let mut json_str = serde_yaml::to_string(&filtered)?;
                     if let Some(max_t) = req.max_tokens {
                         let char_limit = max_t * 4;
                         if json_str.len() > char_limit {
@@ -1305,7 +1305,7 @@ impl McpTool for ManageCheckpointHandler {
                 for s in snaps {
                     list.push(serde_json::json!({"type": "snapshot", "id": s.id, "description": s.description, "namespace": s.namespace}));
                 }
-                Ok(serde_json::to_string_pretty(&list)?)
+                Ok(serde_yaml::to_string(&list)?)
             }
             CheckpointAction::Delete => {
                 let name = req.name_or_id.ok_or_else(|| {
@@ -1427,7 +1427,7 @@ impl McpTool for QueryLineageHandler {
             "timeline": timeline
         });
 
-        Ok(serde_json::to_string_pretty(&res)?)
+        Ok(serde_yaml::to_string(&res)?)
     }
 }
 
@@ -1486,7 +1486,7 @@ impl McpTool for GetNextActionableTasksHandler {
             "tasks": actionable
         });
 
-        Ok(serde_json::to_string_pretty(&res)?)
+        Ok(serde_yaml::to_string(&res)?)
     }
 }
 
@@ -1555,7 +1555,7 @@ impl McpTool for HypothesesHandler {
                     })
                     .collect();
 
-                Ok(serde_json::to_string_pretty(&filtered)?)
+                Ok(serde_yaml::to_string(&filtered)?)
             }
         }
     }
@@ -1634,7 +1634,7 @@ impl McpTool for GetPreflightContextHandler {
             "recent_activities": recent_activities
         });
 
-        Ok(serde_json::to_string_pretty(&preflight)?)
+        Ok(serde_yaml::to_string(&preflight)?)
     }
 }
 
@@ -1742,7 +1742,7 @@ impl McpTool for AgentSignalsHandler {
                         .collect::<Vec<_>>()
                 });
 
-                Ok(serde_json::to_string_pretty(&filtered)?)
+                Ok(serde_yaml::to_string(&filtered)?)
             }
         }
     }