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/notes.rs

@@ -60,7 +60,7 @@ impl McpTool for HandoffMemosHandler {
                             }
                         })
                         .collect();
-                    Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                    Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                 })?;
                 Ok(data)
             }