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/delegate-task/background-continuation.ts

@@ -4,6 +4,7 @@ import { publishToolMetadata } from "../../features/tool-metadata-store"
 import { formatDetailedError } from "./error-formatting"
 import { getSessionTools } from "../../shared/session-tools-store"
 import { buildTaskMetadataBlock } from "../../features/tool-metadata-store/task-metadata-contract"
+import { resolveMetadataModel } from "./resolve-metadata-model"
 import { getTaskID } from "./task-id"
 
 export async function executeBackgroundContinuation(
@@ -42,7 +43,7 @@ export async function executeBackgroundContinuation(
         backgroundTaskId: task.id,
         sessionId: task.sessionID,
         command: args.command,
-        model: task.model ? { providerID: task.model.providerID, modelID: task.model.modelID } : undefined,
+        model: resolveMetadataModel(task.model, parentContext.model),
       },
     }
     await publishToolMetadata(ctx, bgContMeta)