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/config/schema/fallback-models.ts

@@ -1,5 +1,25 @@
 import { z } from "zod"
 
-export const FallbackModelsSchema = z.union([z.string(), z.array(z.string())])
+export const FallbackModelObjectSchema = z.object({
+  model: z.string(),
+  variant: z.string().optional(),
+  reasoningEffort: z.enum(["none", "minimal", "low", "medium", "high", "xhigh"]).optional(),
+  temperature: z.number().min(0).max(2).optional(),
+  top_p: z.number().min(0).max(1).optional(),
+  maxTokens: z.number().optional(),
+  thinking: z
+    .object({
+      type: z.enum(["enabled", "disabled"]),
+      budgetTokens: z.number().optional(),
+    })
+    .optional(),
+})
+
+export type FallbackModelObject = z.infer<typeof FallbackModelObjectSchema>
+
+export const FallbackModelsSchema = z.union([
+  z.string(),
+  z.array(z.union([z.string(), FallbackModelObjectSchema])),
+])
 
 export type FallbackModels = z.infer<typeof FallbackModelsSchema>