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-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