fix: respect user-configured agent models over system defaults

When user explicitly configures an agent model in oh-my-opencode.json,
that model should take priority over the active model in OpenCode's config
(which may just be the system default, not a deliberate UI selection).

This fixes the issue where user-configured models from plugin providers
(e.g., google/antigravity-*) were being overridden by the fallback chain
because config.model was being passed as uiSelectedModel regardless of
whether the user had an explicit config.

The fix:
- Only pass uiSelectedModel when there's no explicit userModel config
- If user has configured a model, let resolveModelPipeline use it directly

Fixes #1573

Co-authored-by: Rishi Vhavle <rishivhavle21@gmail.com>

src/agents/utils.ts

@@ -304,7 +304,7 @@ export async function createBuiltinAgents(
      const isPrimaryAgent = isFactory(source) && source.mode === "primary"
 
     const resolution = applyModelResolution({
-      uiSelectedModel: isPrimaryAgent ? uiSelectedModel : undefined,
+      uiSelectedModel: (isPrimaryAgent && !override?.model) ? uiSelectedModel : undefined,
       userModel: override?.model,
       requirement,
       availableModels,
@@ -356,7 +356,7 @@ export async function createBuiltinAgents(
 
    if (!disabledAgents.includes("sisyphus") && meetsSisyphusAnyModelRequirement) {
     let sisyphusResolution = applyModelResolution({
-      uiSelectedModel,
+      uiSelectedModel: sisyphusOverride?.model ? undefined : uiSelectedModel,
       userModel: sisyphusOverride?.model,
       requirement: sisyphusRequirement,
       availableModels,
@@ -454,7 +454,7 @@ export async function createBuiltinAgents(
       const atlasRequirement = AGENT_MODEL_REQUIREMENTS["atlas"]
 
       const atlasResolution = applyModelResolution({
-        uiSelectedModel,
+        uiSelectedModel: orchestratorOverride?.model ? undefined : uiSelectedModel,
         userModel: orchestratorOverride?.model,
         requirement: atlasRequirement,
         availableModels,