fix(schema): preserve custom agent overrides via catchall

AgentOverridesSchema silently strips custom agent keys during Zod
parsing because only 14 built-in names are explicitly defined. Add
.catchall(AgentOverrideConfigSchema.optional()) so user-defined agent
configs survive validation and reach downstream consumers like
resolveModelAndFallbackChain().

Fixes #3229.

src/config/schema/agent-overrides.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, test } from "bun:test"
+import { AgentOverridesSchema } from "./agent-overrides"
+
+describe("AgentOverridesSchema", () => {
+  test("preserves custom agent keys after parsing", () => {
+    const input = {
+      sisyphus: { model: "anthropic/claude-opus-4-6" },
+      "technical-writer": {
+        model: "anthropic/claude-sonnet-4-6",
+        temperature: 0.3,
+        prompt_append: "You are a technical writer.",
+      },
+    }
+
+    const result = AgentOverridesSchema.safeParse(input)
+
+    expect(result.success).toBe(true)
+    if (result.success) {
+      expect(result.data.sisyphus).toBeDefined()
+      expect(result.data["technical-writer"]).toBeDefined()
+      expect(result.data["technical-writer"]?.model).toBe("anthropic/claude-sonnet-4-6")
+      expect(result.data["technical-writer"]?.temperature).toBe(0.3)
+    }
+  })
+
+  test("validates custom agent keys against AgentOverrideConfigSchema", () => {
+    const input = {
+      "custom-agent": {
+        model: "provider/model",
+        temperature: 5, // invalid: max is 2
+      },
+    }
+
+    const result = AgentOverridesSchema.safeParse(input)
+
+    expect(result.success).toBe(false)
+  })
+})

src/config/schema/agent-overrides.ts

@@ -72,7 +72,7 @@ export const AgentOverridesSchema = z.object({
   explore: AgentOverrideConfigSchema.optional(),
   "multimodal-looker": AgentOverrideConfigSchema.optional(),
   atlas: AgentOverrideConfigSchema.optional(),
-})
+}).catchall(AgentOverrideConfigSchema.optional())
 
 export type AgentOverrideConfig = z.infer<typeof AgentOverrideConfigSchema>
 export type AgentOverrides = z.infer<typeof AgentOverridesSchema>