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/category-resolver.ts

@@ -7,14 +7,16 @@ import { SISYPHUS_JUNIOR_AGENT } from "./sisyphus-junior-agent"
 import { resolveCategoryConfig } from "./categories"
 import { parseModelString } from "./model-string-parser"
 import { CATEGORY_MODEL_REQUIREMENTS } from "../../shared/model-requirements"
-import { normalizeFallbackModels } from "../../shared/model-resolver"
-import { buildFallbackChainFromModels } from "../../shared/fallback-chain-from-models"
+import { normalizeFallbackModels, flattenToFallbackModelStrings } from "../../shared/model-resolver"
+import { buildFallbackChainFromModels, findMostSpecificFallbackEntry } from "../../shared/fallback-chain-from-models"
 import { getAvailableModelsForDelegateTask } from "./available-models"
 import { resolveModelForDelegateTask } from "./model-selection"
 
+import type { DelegatedModelConfig } from "./types"
+
 export interface CategoryResolutionResult {
   agentToUse: string
-  categoryModel: { providerID: string; modelID: string; variant?: string } | undefined
+  categoryModel: DelegatedModelConfig | undefined
   categoryPromptAppend: string | undefined
   maxPromptTokens?: number
   modelInfo: ModelFallbackInfo | undefined
@@ -84,8 +86,9 @@ Available categories: ${allCategoryNames}`,
   const normalizedConfiguredFallbackModels = normalizeFallbackModels(resolved.config.fallback_models)
   let actualModel: string | undefined
   let modelInfo: ModelFallbackInfo | undefined
-  let categoryModel: { providerID: string; modelID: string; variant?: string } | undefined
+  let categoryModel: DelegatedModelConfig | undefined
   let isModelResolutionSkipped = false
+  let fallbackEntry: FallbackEntry | undefined
 
   const overrideModel = sisyphusJuniorModel
   const explicitCategoryModel = userCategories?.[args.category!]?.model
@@ -108,7 +111,7 @@ Available categories: ${allCategoryNames}`,
   } else {
     const resolution = resolveModelForDelegateTask({
       userModel: explicitCategoryModel ?? overrideModel,
-      userFallbackModels: normalizedConfiguredFallbackModels,
+      userFallbackModels: flattenToFallbackModelStrings(normalizedConfiguredFallbackModels),
       categoryDefaultModel: resolved.model,
       isUserConfiguredCategoryModel: resolved.isUserConfiguredModel,
       fallbackChain: requirement.fallbackChain,
@@ -119,7 +122,8 @@ Available categories: ${allCategoryNames}`,
     if (resolution && "skipped" in resolution) {
       isModelResolutionSkipped = true
     } else if (resolution) {
-      const { model: resolvedModel, variant: resolvedVariant } = resolution
+      const { model: resolvedModel, variant: resolvedVariant, fallbackEntry: resolvedFallbackEntry } = resolution
+      fallbackEntry = resolvedFallbackEntry
       actualModel = resolvedModel
 
       if (!parseModelString(actualModel)) {
@@ -198,6 +202,26 @@ Available categories: ${categoryNames.join(", ")}`,
     defaultProviderID,
   )
 
+  // Apply per-model settings from the source that provided the match:
+  // 1. fallbackEntry from resolver (built-in chain match) — exact, no lookup needed
+  // 2. configuredFallbackChain (user's fallback_models) — prefix match against user config
+  const effectiveEntry = fallbackEntry
+    ?? (categoryModel && configuredFallbackChain
+      ? findMostSpecificFallbackEntry(categoryModel.providerID, categoryModel.modelID, configuredFallbackChain)
+      : undefined)
+
+  if (categoryModel && effectiveEntry) {
+    categoryModel = {
+      ...categoryModel,
+      variant: userCategories?.[args.category!]?.variant ?? effectiveEntry.variant ?? categoryModel.variant,
+      reasoningEffort: effectiveEntry.reasoningEffort,
+      temperature: effectiveEntry.temperature,
+      top_p: effectiveEntry.top_p,
+      maxTokens: effectiveEntry.maxTokens,
+      thinking: effectiveEntry.thinking,
+    }
+  }
+
   return {
     agentToUse: SISYPHUS_JUNIOR_AGENT,
     categoryModel,