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/features/background-agent/manager.ts

@@ -567,6 +567,15 @@ export class BackgroundManager {
         parentID: input.parentSessionId,
         title: `${input.description} (@${input.agent} subagent)`,
         ...(input.sessionPermission ? { permission: input.sessionPermission } : {}),
+        ...(input.model
+          ? {
+              model: {
+                id: input.model.modelID,
+                providerID: input.model.providerID,
+                ...(input.model.variant ? { variant: input.model.variant } : {}),
+              },
+            }
+          : {}),
       } as Record<string, unknown>,
       query: {
         directory: parentDirectory,