feat(config): support object-style fallback_models with per-model settings

Add support for object-style entries in fallback_models arrays, enabling
per-model configuration of variant, reasoningEffort, temperature, top_p,
maxTokens, and thinking settings.

- Zod schema for FallbackModelObject with full validation
- normalizeFallbackModels() and flattenToFallbackModelStrings() utilities
- Provider-agnostic model resolution pipeline with fallback chain
- Session prompt params state management
- Fallback chain construction with prefix-match lookup
- Integration across delegate-task, background-agent, and plugin layers

src/agents/types.ts

@@ -123,7 +123,7 @@ export type AgentName = BuiltinAgentName;
 export type AgentOverrideConfig = Partial<AgentConfig> & {
   prompt_append?: string;
   variant?: string;
-  fallback_models?: string | string[];
+  fallback_models?: string | (string | import("../config/schema/fallback-models").FallbackModelObject)[];
 };
 
 export type AgentOverrides = Partial<