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/config/schema.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, test } from "bun:test"
-import { AgentOverrideConfigSchema, BuiltinCategoryNameSchema, OhMyOpenCodeConfigSchema } from "./schema"
+import { AgentOverrideConfigSchema, BuiltinCategoryNameSchema, CategoryConfigSchema, OhMyOpenCodeConfigSchema } from "./schema"
 
 describe("disabled_mcps schema", () => {
   test("should accept built-in MCP names", () => {
@@ -174,6 +174,33 @@ describe("AgentOverrideConfigSchema", () => {
     })
   })
 
+  describe("variant field", () => {
+    test("accepts variant as optional string", () => {
+      // #given
+      const config = { variant: "high" }
+
+      // #when
+      const result = AgentOverrideConfigSchema.safeParse(config)
+
+      // #then
+      expect(result.success).toBe(true)
+      if (result.success) {
+        expect(result.data.variant).toBe("high")
+      }
+    })
+
+    test("rejects non-string variant", () => {
+      // #given
+      const config = { variant: 123 }
+
+      // #when
+      const result = AgentOverrideConfigSchema.safeParse(config)
+
+      // #then
+      expect(result.success).toBe(false)
+    })
+  })
+
   describe("skills field", () => {
     test("accepts skills as optional string array", () => {
       // #given
@@ -303,6 +330,33 @@ describe("AgentOverrideConfigSchema", () => {
   })
 })
 
+describe("CategoryConfigSchema", () => {
+  test("accepts variant as optional string", () => {
+    // #given
+    const config = { model: "openai/gpt-5.2", variant: "xhigh" }
+
+    // #when
+    const result = CategoryConfigSchema.safeParse(config)
+
+    // #then
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.variant).toBe("xhigh")
+    }
+  })
+
+  test("rejects non-string variant", () => {
+    // #given
+    const config = { model: "openai/gpt-5.2", variant: 123 }
+
+    // #when
+    const result = CategoryConfigSchema.safeParse(config)
+
+    // #then
+    expect(result.success).toBe(false)
+  })
+})
+
 describe("BuiltinCategoryNameSchema", () => {
   test("accepts all builtin category names", () => {
     // #given

src/config/schema.ts

@@ -97,6 +97,7 @@ export const BuiltinCommandNameSchema = z.enum([
 export const AgentOverrideConfigSchema = z.object({
   /** @deprecated Use `category` instead. Model is inherited from category defaults. */
   model: z.string().optional(),
+  variant: z.string().optional(),
   /** Category name to inherit model and other settings from CategoryConfig */
   category: z.string().optional(),
   /** Skill names to inject into agent prompt */
@@ -153,6 +154,7 @@ export const SisyphusAgentConfigSchema = z.object({
 
 export const CategoryConfigSchema = z.object({
   model: z.string(),
+  variant: z.string().optional(),
   temperature: z.number().min(0).max(2).optional(),
   top_p: z.number().min(0).max(1).optional(),
   maxTokens: z.number().optional(),