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/index.ts

@@ -35,7 +35,7 @@ export * from "./agent-tool-restrictions"
 export * from "./model-requirements"
 export * from "./model-resolver"
 export { normalizeModel, normalizeModelID } from "./model-normalization"
-export { normalizeFallbackModels } from "./model-resolver"
+export { normalizeFallbackModels, flattenToFallbackModelStrings } from "./model-resolver"
 export { resolveModelPipeline } from "./model-resolution-pipeline"
 export type {
   ModelResolutionRequest,