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/agents/types.ts

@@ -76,6 +76,7 @@ export type AgentName = BuiltinAgentName
 
 export type AgentOverrideConfig = Partial<AgentConfig> & {
   prompt_append?: string
+  variant?: string
 }
 
 export type AgentOverrides = Partial<Record<OverridableAgentName, AgentOverrideConfig>>