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/subagent-resolver.ts

@@ -1,11 +1,12 @@
 import type { DelegateTaskArgs } from "./types"
 import type { ExecutorContext } from "./executor-types"
+import type { DelegatedModelConfig } from "./types"
 import { isPlanFamily } from "./constants"
 import { SISYPHUS_JUNIOR_AGENT } from "./sisyphus-junior-agent"
 import { normalizeModelFormat } from "../../shared/model-format-normalizer"
 import { AGENT_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 { getAgentDisplayName, getAgentConfigKey } from "../../shared/agent-display-names"
 import { normalizeSDKResponse } from "../../shared"
 import { log } from "../../shared/logger"
@@ -17,9 +18,8 @@ export async function resolveSubagentExecution(
   args: DelegateTaskArgs,
   executorCtx: ExecutorContext,
   parentAgent: string | undefined,
-  categoryExamples: string,
-  inheritedModel?: string
-): Promise<{ agentToUse: string; categoryModel: { providerID: string; modelID: string; variant?: string } | undefined; fallbackChain?: FallbackEntry[]; error?: string }> {
+  categoryExamples: string
+): Promise<{ agentToUse: string; categoryModel: DelegatedModelConfig | undefined; fallbackChain?: FallbackEntry[]; error?: string }> {
   const { client, agentOverrides, userCategories } = executorCtx
 
   if (!args.subagent_type?.trim()) {
@@ -49,7 +49,7 @@ Create the work plan directly - that's your job as the planning agent.`,
   }
 
   let agentToUse = agentName
-  let categoryModel: { providerID: string; modelID: string; variant?: string } | undefined
+  let categoryModel: DelegatedModelConfig | undefined
   let fallbackChain: FallbackEntry[] | undefined = undefined
 
   try {
@@ -117,8 +117,8 @@ Create the work plan directly - that's your job as the planning agent.`,
         : undefined
 
       const resolution = resolveModelForDelegateTask({
-        userModel: agentOverride?.model ?? inheritedModel,
-        userFallbackModels: normalizedAgentFallbackModels,
+        userModel: agentOverride?.model,
+        userFallbackModels: flattenToFallbackModelStrings(normalizedAgentFallbackModels),
         categoryDefaultModel: matchedAgentModelStr,
         fallbackChain: agentRequirement?.fallbackChain,
         availableModels,
@@ -141,6 +141,25 @@ Create the work plan directly - that's your job as the planning agent.`,
         defaultProviderID,
       )
       fallbackChain = configuredFallbackChain ?? agentRequirement?.fallbackChain
+
+      // Apply per-model settings: prefer resolver's exact entry, fall back to prefix match on user config
+      const resolvedFallbackEntry = (resolution && !('skipped' in resolution)) ? resolution.fallbackEntry : undefined
+      const effectiveEntry = resolvedFallbackEntry
+        ?? (categoryModel && fallbackChain
+          ? findMostSpecificFallbackEntry(categoryModel.providerID, categoryModel.modelID, fallbackChain)
+          : undefined)
+
+      if (categoryModel && effectiveEntry) {
+        categoryModel = {
+          ...categoryModel,
+          variant: agentOverride?.variant ?? effectiveEntry.variant ?? categoryModel.variant,
+          reasoningEffort: effectiveEntry.reasoningEffort,
+          temperature: effectiveEntry.temperature,
+          top_p: effectiveEntry.top_p,
+          maxTokens: effectiveEntry.maxTokens,
+          thinking: effectiveEntry.thinking,
+        }
+      }
     }
 
     if (!categoryModel && matchedAgent.model) {