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.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>