fix: preserve user-selected variant on first message instead of overriding with fallback chain default

First message variant gate was unconditionally overwriting message.variant
with the fallback chain value (e.g. 'medium' for Hephaestus), ignoring
any variant the user had already selected via OpenCode UI.

Now checks message.variant === undefined before applying the resolved
variant, matching the behavior already used for subsequent messages.

Closes #1861

src/plugin/chat-message.ts

@@ -56,12 +56,14 @@ export function createChatMessageHandler(args: {
     const message = output.message
 
     if (firstMessageVariantGate.shouldOverride(input.sessionID)) {
-      const variant =
-        input.model && input.agent
-          ? resolveVariantForModel(pluginConfig, input.agent, input.model)
-          : resolveAgentVariant(pluginConfig, input.agent)
-      if (variant !== undefined) {
-        message["variant"] = variant
+      if (message["variant"] === undefined) {
+        const variant =
+          input.model && input.agent
+            ? resolveVariantForModel(pluginConfig, input.agent, input.model)
+            : resolveAgentVariant(pluginConfig, input.agent)
+        if (variant !== undefined) {
+          message["variant"] = variant
+        }
       }
       firstMessageVariantGate.markApplied(input.sessionID)
     } else {