fix: pass resolved model to session.create so sub-agent sessions use the correct model

Sub-agent sessions created via the task tool or call_omo_agent tool were
being created without a model. The resolved model was only passed as a
promptAsync body override, which opencode core ignores, causing fallback
to the client's system default model (often a reasoning model that hangs).

OpenCode's session.create API supports model: { id, providerID, variant }
at creation time. This fix ensures the resolved category/agent model is
passed during session creation across all paths:

- createSyncSession() (delegate task / category-based)
- createOrGetSession() (call_omo_agent / direct agent calls)
- BackgroundManager.startTask() (background tasks)

This guarantees each sub-agent session is created with the correct model
regardless of whether promptAsync honors its model override.

src/tools/delegate-task/sync-session-creator.ts

@@ -1,9 +1,16 @@
 import type { OpencodeClient } from "./types"
+import type { DelegatedModelConfig } from "../../shared/model-resolution-types"
 import { QUESTION_DENIED_SESSION_PERMISSION } from "../../shared/question-denied-session-permission"
 
 export async function createSyncSession(
   client: OpencodeClient,
-  input: { parentSessionID: string; agentToUse: string; description: string; defaultDirectory: string }
+  input: {
+    parentSessionID: string
+    agentToUse: string
+    description: string
+    defaultDirectory: string
+    categoryModel?: DelegatedModelConfig
+  }
 ): Promise<{ ok: true; sessionID: string; parentDirectory: string } | { ok: false; error: string }> {
   const parentSession = client.session.get
     ? await client.session.get({ path: { id: input.parentSessionID } }).catch(() => null)
@@ -15,6 +22,15 @@ export async function createSyncSession(
       parentID: input.parentSessionID,
       title: `${input.description} (@${input.agentToUse} subagent)`,
       permission: QUESTION_DENIED_SESSION_PERMISSION,
+      ...(input.categoryModel
+        ? {
+            model: {
+              id: input.categoryModel.modelID,
+              providerID: input.categoryModel.providerID,
+              ...(input.categoryModel.variant ? { variant: input.categoryModel.variant } : {}),
+            },
+          }
+        : {}),
     } as Record<string, unknown>,
     query: {
       directory: parentDirectory,

src/tools/delegate-task/sync-task.ts

@@ -64,6 +64,7 @@ export async function executeSyncTask(
       agentToUse,
       description: args.description,
       defaultDirectory: directory,
+      categoryModel,
     })
 
     if (!createSessionResult.ok) {
@@ -227,6 +228,7 @@ export async function executeSyncTask(
             agentToUse,
             description: args.description,
             defaultDirectory: directory,
+            categoryModel: nextFallbackModel,
           })
           if (!retrySessionResult.ok) {
             return retrySessionResult.error