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/tools/delegate-task/unstable-agent-task.ts

@@ -1,4 +1,4 @@
-import type { DelegateTaskArgs, ToolContextWithMetadata } from "./types"
+import type { DelegateTaskArgs, ToolContextWithMetadata, DelegatedModelConfig } from "./types"
 import type { ExecutorContext, ParentContext, SessionMessage } from "./executor-types"
 import { DEFAULT_SYNC_POLL_TIMEOUT_MS, getTimingConfig } from "./timing"
 import { buildTaskPrompt } from "./prompt-builder"
@@ -16,7 +16,7 @@ export async function executeUnstableAgentTask(
   executorCtx: ExecutorContext,
   parentContext: ParentContext,
   agentToUse: string,
-  categoryModel: { providerID: string; modelID: string; variant?: string } | undefined,
+  categoryModel: DelegatedModelConfig | undefined,
   systemContent: string | undefined,
   actualModel: string | undefined
 ): Promise<string> {