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/shared/model-requirements.ts

@@ -2,6 +2,11 @@ export type FallbackEntry = {
   providers: string[];
   model: string;
   variant?: string; // Entry-specific variant (e.g., GPT→high, Opus→max)
+  reasoningEffort?: string;
+  temperature?: number;
+  top_p?: number;
+  maxTokens?: number;
+  thinking?: { type: "enabled" | "disabled"; budgetTokens?: number };
 };
 
 export type ModelRequirement = {