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/call-omo-agent/sync-executor.ts

@@ -73,7 +73,7 @@ export async function executeSync(
   let appliedFallbackChain = false
 
   try {
-    const session = await deps.createOrGetSession(args, toolContext, ctx)
+    const session = await deps.createOrGetSession(args, toolContext, ctx, model)
     sessionID = session.sessionID
     createdSessionForExecution = session.isNew
     subagentSessions.add(sessionID)