fix(delegate-task): align metadata with opencode core task tool

Match opencode core 'task' tool behavior for metadata consistency:

1. Model fallback: When categoryModel/task.model/resumeModel is undefined,

   fall back to parentContext.model so subagent metadata always includes

   model info. Thread parentContext into executeSyncContinuation for parity.

2. Task ID consistency: unstable-agent-task was missing taskId and

   backgroundTaskId in metadata. background_output used inconsistent

   snake_case 'task_id' vs camelCase 'taskId' elsewhere. Standardize on

   camelCase: taskId = sessionID (resume id), backgroundTaskId = bg task id.

Update text output blocks to use buildTaskMetadataBlock helper.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/background-task/create-background-output.metadata.test.ts

@@ -59,7 +59,7 @@ describe("createBackgroundOutput metadata", () => {
         agent: "test-agent",
         category: undefined,
         description: "background task",
-        task_id: "task-1",
+        backgroundTaskId: "task-1",
       },
     })
 

src/tools/background-task/create-background-output.ts

@@ -66,11 +66,11 @@ export function createBackgroundOutput(manager: BackgroundOutputManager, client:
         const meta = {
           title: formatResolvedTitle(task),
           metadata: {
-            task_id: task.id,
+            backgroundTaskId: task.id,
             agent: task.agent,
             category: task.category,
             description: task.description,
-            ...(task.sessionID ? { sessionId: task.sessionID } : {}),
+            ...(task.sessionID ? { sessionId: task.sessionID, taskId: task.sessionID } : {}),
           } as Record<string, unknown>,
         }
         await publishToolMetadata(ctx, meta)