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/tasks.rs

@@ -297,12 +297,12 @@ impl McpTool for TasksHandler {
                             _ => serde_json::json!({ "id": t.id, "title": t.title, "status": t.status, "description": t.description, "git_branch": t.git_branch }),
                         })
                         .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;
                         while json_str.len() > char_limit && !filtered.is_empty() {
                             filtered.pop();
-                            json_str = serde_json::to_string(&filtered)?;
+                            json_str = serde_yaml::to_string(&filtered)?;
                         }
                     }
                     Ok::<String, crate::error::AppError>(json_str)
@@ -514,7 +514,7 @@ impl McpTool for MilestonesHandler {
                             }
                         })
                         .collect();
-                    Ok::<String, crate::error::AppError>(serde_json::to_string(&filtered)?)
+                    Ok::<String, crate::error::AppError>(serde_yaml::to_string(&filtered)?)
                 })?;
                 Ok(data)
             }