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.test.ts

@@ -0,0 +1,83 @@
+import { describe, expect, test } from "bun:test"
+import type { OhMyOpenCodeConfig } from "../config"
+import { applyAgentVariant, resolveAgentVariant } from "./agent-variant"
+
+describe("resolveAgentVariant", () => {
+  test("returns undefined when agent name missing", () => {
+    // #given
+    const config = {} as OhMyOpenCodeConfig
+
+    // #when
+    const variant = resolveAgentVariant(config)
+
+    // #then
+    expect(variant).toBeUndefined()
+  })
+
+  test("returns agent override variant", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { variant: "low" },
+      },
+    } as OhMyOpenCodeConfig
+
+    // #when
+    const variant = resolveAgentVariant(config, "Sisyphus")
+
+    // #then
+    expect(variant).toBe("low")
+  })
+
+  test("returns category variant when agent uses category", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { category: "ultrabrain" },
+      },
+      categories: {
+        ultrabrain: { model: "openai/gpt-5.2", variant: "xhigh" },
+      },
+    } as OhMyOpenCodeConfig
+
+    // #when
+    const variant = resolveAgentVariant(config, "Sisyphus")
+
+    // #then
+    expect(variant).toBe("xhigh")
+  })
+})
+
+describe("applyAgentVariant", () => {
+  test("sets variant when message is undefined", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { variant: "low" },
+      },
+    } as OhMyOpenCodeConfig
+    const message: { variant?: string } = {}
+
+    // #when
+    applyAgentVariant(config, "Sisyphus", message)
+
+    // #then
+    expect(message.variant).toBe("low")
+  })
+
+  test("does not override existing variant", () => {
+    // #given
+    const config = {
+      agents: {
+        Sisyphus: { variant: "low" },
+      },
+    } as OhMyOpenCodeConfig
+    const message = { variant: "max" }
+
+    // #when
+    applyAgentVariant(config, "Sisyphus", message)
+
+    // #then
+    expect(message.variant).toBe("max")
+  })
+})