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/unstable-agent-task.ts

@@ -9,6 +9,8 @@ import { formatDetailedError } from "./error-formatting"
 import { getSessionTools } from "../../shared/session-tools-store"
 import { normalizeSDKResponse } from "../../shared"
 import { QUESTION_DENIED_SESSION_PERMISSION } from "../../shared/question-denied-session-permission"
+import { resolveMetadataModel } from "./resolve-metadata-model"
+import { buildTaskMetadataBlock } from "../../features/tool-metadata-store/task-metadata-contract"
 
 export async function executeUnstableAgentTask(
   args: DelegateTaskArgs,
@@ -75,9 +77,11 @@ export async function executeUnstableAgentTask(
         load_skills: args.load_skills,
         description: args.description,
         run_in_background: args.run_in_background,
+        taskId: sessionID,
+        backgroundTaskId: task.id,
         sessionId: sessionID,
         command: args.command,
-        model: categoryModel ? { providerID: categoryModel.providerID, modelID: categoryModel.modelID } : undefined,
+        model: resolveMetadataModel(categoryModel, parentContext.model),
       },
     }
     await publishToolMetadata(ctx, bgTaskMeta)
@@ -147,9 +151,13 @@ Model: ${actualModel}
 
 The task session may contain partial results.
 
-<task_metadata>
-session_id: ${sessionID}
-</task_metadata>`
+${buildTaskMetadataBlock({
+        sessionId: sessionID,
+        taskId: sessionID,
+        backgroundTaskId: task.id,
+        agent: agentToUse,
+        category: args.category,
+      })}`
     }
 
     if (!completedDuringMonitoring) {
@@ -167,9 +175,13 @@ Model: ${actualModel}
 
 The task session may still contain partial results.
 
-<task_metadata>
-session_id: ${sessionID}
-</task_metadata>`
+${buildTaskMetadataBlock({
+        sessionId: sessionID,
+        taskId: sessionID,
+        backgroundTaskId: task.id,
+        agent: agentToUse,
+        category: args.category,
+      })}`
     }
 
     const messagesResult = await client.session.messages({ path: { id: sessionID } })
@@ -217,9 +229,13 @@ RESULT:
 
 ${textContent || "(No text output)"}
 
-<task_metadata>
-session_id: ${sessionID}
-</task_metadata>`
+${buildTaskMetadataBlock({
+      sessionId: sessionID,
+      taskId: sessionID,
+      backgroundTaskId: task.id,
+      agent: agentToUse,
+      category: args.category,
+    })}`
   } catch (error) {
     if (!cleanupReason) {
       cleanupReason = "exception"