feat(config): add model variant support

Allow optional model variant config for agents and categories.
Propagate category variants into task model payloads so
category-driven runs inherit provider-specific variants.

Closes: #647

src/shared/agent-variant.ts

@@ -0,0 +1,40 @@
+import type { OhMyOpenCodeConfig } from "../config"
+
+export function resolveAgentVariant(
+  config: OhMyOpenCodeConfig,
+  agentName?: string
+): string | undefined {
+  if (!agentName) {
+    return undefined
+  }
+
+  const agentOverrides = config.agents as
+    | Record<string, { variant?: string; category?: string }>
+    | undefined
+  const agentOverride = agentOverrides?.[agentName]
+  if (!agentOverride) {
+    return undefined
+  }
+
+  if (agentOverride.variant) {
+    return agentOverride.variant
+  }
+
+  const categoryName = agentOverride.category
+  if (!categoryName) {
+    return undefined
+  }
+
+  return config.categories?.[categoryName]?.variant
+}
+
+export function applyAgentVariant(
+  config: OhMyOpenCodeConfig,
+  agentName: string | undefined,
+  message: { variant?: string }
+): void {
+  const variant = resolveAgentVariant(config, agentName)
+  if (variant !== undefined && message.variant === undefined) {
+    message.variant = variant
+  }
+}