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/git.rs

@@ -87,7 +87,7 @@ impl McpTool for GetActiveWorktreeContextHandler {
             .await
             .map_err(|e| crate::error::AppError::Internal(format!("Task panic: {}", e)))??;
 
-        Ok::<String, crate::error::AppError>(serde_json::to_string_pretty(&result)?)
+        Ok::<String, crate::error::AppError>(serde_yaml::to_string(&result)?)
     }
 }
 
@@ -168,7 +168,7 @@ impl McpTool for QueryGitDiffsHandler {
             matches.extend(git_matches);
         }
 
-        Ok(serde_json::to_string_pretty(&matches)?)
+        Ok(serde_yaml::to_string(&matches)?)
     }
 }